What would make sailing difficult?

Question

I'm looking to write a story where the world is a lot of islands separated by rather small water expanses (about the width of the Mediterranean or maybe that and a half) and I am struggling to find a reason to have a guild control travel, a bit like in Dune with space travel.

Essentially, I'm looking to bring the difficulty of space travel from Star Wars or Dune into a world that is ocean-based.

My initial thought was to have it be very expensive, so some kind of mercantile aspect needed to be part of each travel, but it seems difficult to explain.

The world I am looking at is somewhat post-apocalyptic. After some disaster in a modern-ish (maybe 1950s) tech level made living difficult, the contact of the planet with some kind of liquid raised the ocean level by several hundred metres and only very few survived. So technology is at a rather weird point, and magic is a thing in that world so any kind of magical navigation would be welcome, but this would not necessarily mean that regular navigation is impossible.

I had considered making the liquid that raised the ocean level be something other than water but it would also affect the rest of the world as if it were poison it would get on land and poison that too, and making it acidic seems difficult considering that islands need to continue existing…

Any ideas on how to give that strong barrier feel requiring specialised help without actually putting things in space?

Answer 1

Massive sea snakes

You can have a species of sea serpents that are ~50 ft long and attack ships thinking they are whales, since they live below where the whales swim they mistake the ships for the bellies of whales and attack them

The merchant guild can have any one of the following:

• magicians that cast wards around the ships that essentially repel the snakes (these don't need to be magical, you can just have some kind of chemical do it if you prefer not using magic)

• expensive submarines that can a) see the snakes coming and warn the ships above and b) potentially attack and drive off / kill the predators with their powerful technology.

Answer 2

Sailing is risky and requires lots of knowledge

Just take a page from history. During the age of sail, specially during its first decades, explorers such as Columbus, Cabral, Magellan etc. had a hell of a bad time navigating the Atlantic. A major reason why they got so filthy rich was because sailing was a high-risk activity with large returns.

Sailing itself is not easy, specially with large vessels. For long distances, piloting knowledge is not enough - you need to plan for the currents and winds you'll face. Make a mistake, and you either bring too few supplies and die for need of them, or carry too many supplies which slow you down and become a logistics problem. Now imagine trying to plan for those without knowing where the currents and winds will take you, and without knowing how strong they are. Magellan and his crew nearly died of starvation when crossing the Pacific because of that.

Having good maps changes everything. Around the 1500's the Portuguese had a practical monopoly on marine trade with the Indies. They had been compiling maps of currents, winds, sandbanks and routes for centuries, but they kept those secret. Then by the end of the 16th century a Dutch dude called Jan Huyghen van Linschoten stole and published those. That had an impact in the 17th century orders of magnitude larger than Wikileaks had in our own time:

He is credited with publishing in Europe important classified information about Asian trade and navigation that was hidden by the Portuguese. In 1596 he published a book, Itinerario (later published as an English edition as Discours of Voyages into Ye East & West Indies) which graphically displayed for the first time in Europe detailed maps of voyages to the East Indies, particularly India. During his stay in Goa, Jan Huygen van Linschoten meticulously copied the top-secret charts page-by-page. Even more crucially, Jan Huygen van Linschoten provided nautical data like currents, deeps, islands and sandbanks, which was absolutely vital for safe navigation, along with coastal depictions to guide the way. The publication of the navigational routes enabled the passage to the East Indies to be opened to trading by the Dutch, French and the English. As a consequence, The Dutch East India Company and British East India Company would break the 16th-century monopoly enjoyed by the Portuguese on trade with the East Indies.

Seriously. You all would be fluent in Portuguese rather than English now if it wasn't for van Linschoten.

Your world can have its own Portugal, maintaining a monopoly on marine trade by maintaining a monopoly on knowledge.

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Edit: maps are not the only secret they could keep. user3445853 made this comment:

The Dutch found the secret to preserving herring in a tasty fashion ("matjes/maatjes herring": remove all intestines except pancreas and pickle in a light brine --- pancreatic enzymes break down various structures in a tasty and stable way), and succeeded in keeping it secret for 300years (from just before 1400), selling the swedes and germans fish caught in their own waters and getting rich. This secret ANY uneducated deckhand could steal and sell, but they didn't succeed [easier steal than a detailed map set!]. So the maritime brotherhood CAN guard a secret.

This is specially interesting because many deaths on sea were caused by scurvy. Just so you know, when you've had scurvy for a while and you're still malnourished, every single scar you have in your body starts bleeding and your teeth start to fall.

So in your world, people can either pay the guild fairly and eat preserved food during their travels, or they can venture on their own not knowing where the wind blows, the directions of currents, and to add insult to injury they will only have biscuits, crackers and rats to eat.

Answer 3

Navigation.

The sea is almost always covered in fog or heavy haze, making navigation by stars and sun a bit problematic.

And ever since the partial Magnetic Reversal (what done in the real civilization!), magnetic compasses just argue with each other, and point to the nearest horseshoe.

The Navigator's Guild knows some dark secret to keeping your orientation and knowing your position on the water, but it is a dark and jealously guarded mystery.

(It's actually just simple old Gyrocompasses for location and direction, and a very very good store of maps including currents, but don't tell anyone!)

Answer 4

The oceans are now not very deep. Deep enough that you couldn't walk through them, but not deep enough to submerge all the rocks that now poke out the water. This makes sailing dangerous as almost any instance you could be knocked into a rock and get a hole in your boat. Depending how post apocalyptic you want, these "rocks" could be the remains of the buildings of the previous civilization. I'm not sure I'd want to try an sail a boat though the flooded remains of major city, where all your seeing is the very tops of tall buildings, and there could easily be buildings just below the surface you can't see.

Answer 5

Hostile intelligences.

Such things are the plot of at least two books I've read in the past... intelligent organisms, alien or otherwise, take exception to human activity in the oceans and take steps to destroy human shipping and economic exploitation of the seas, extending as far as assaults on dry land by suitably engineered organisms.

In one of the books the intelligences used nuclear-powered heating devices to attack the ice caps, in the other seabed clathrate deposits were disrupted to trigger an underwater landslide which in turn caused a massive surface tsunami.

In one story the intelligences were apparently attacked and killed with a novel weapon, but in the other an uneasy truce arose at the very end. That sort of state of affairs seems like it could also arise in your world, with only the guild having made the appropriate overtures to the dwellers in the deep. Intrusions into the sea by non-guild ships might irritate the dwellers who may impose sanctions upon the guild (to encourage humans to police their own affairs), to take action on the treaty-breakers by sinking their ships or worse, to attack the landmasses the intruding ships came from.

The latter might make for an interesting dynamic where no-one other than the guild has seafront settlements, because no-one else wants to find themselves swept away by a freak wave or more unpleasant things crawling up steathily out of the sea at night...

(for those of you who might be interested in what the books are, and don't care about the spoilery nature of the text above, here you go)

The Swarm, by Frank Schätzing, and The Kraken Wakes by John Wyndham. The latter is much shorter than the former, if you were thinking of hunting either of them down.

Answer 6

Zero Wind

...means you'd have to row (assuming engines are not available), and there's a practical limit to how far you can reasonably get.

Answer 7

Your disaster put enough dust in orbit to turn the night sky pitch black

You can navigate during the day by using the time and the angle to the sun to get bearing and location. You can use this for short distance travel of duration less than 1 day.

However at night, it's pitch black. No stars. No moon. Nothing to navigate by. You can not get your position accurately for 50% of the time.

Combine this with some magnetic reefs that make compasses unreliable, deep oceans where it's impossible to drop anchor and wait out the night, and uneven prevailing winds that can subtly turn you, and nobody will be able to cross the water until they develop speed boats that can do the crossing in one day, or flight.

Answer 8

Attack the sails themselves by Mothra

OK, maybe not Mothra, but having a new mutation of moths that absolutely loves to eat the cloth used to make sails when they are salty, i.e. as soon as you start using them.

With loss of the ability to make canvas out of Dacron, etc. only organic material remain somewhat practical (relatively inexpensive and functional). However, if you go through a dozen sets of canvas during a short voyage, it will raise the materials and labor cost and reduce cargo capacity considerably.

This would be a nice variation in that fresh-water sailing could be cheap when ocean sailing is not. Clothing that gets sweaty would be a problem too.

The controlling guild could simply be one that basically had a monopoly status on making the sails that could afford to drive competitors out of business.

Answer 9

There are a lot of closely spaced reefs which require careful navigation. Add to this unpredictable weather events which make the seas very choppy and hazardous.

With so few people remaining, no-one has the ability to navigate by celestial or solar means.

Take some ideas from Jason and the Argonauts and have something similar to Sirens trying to draw in unwary sailors so their vessels and cargoes can be stolen and any passengers and crew can be taken in slavery.

Less dangerous waterways could be controlled by "nobles" or despots who apply tolls for safe passage through such waterways forcing some people to avoid them and use more dangerous routes.

Currents in narrow waterways can be very treacherous.

Answer 10

Shortages of raw materials

For most of human history, navigation by sea required specific raw materials, mainly wood (and lots of it), but also hemp, tar etc. Planks can be cut from shorter trees (and even these need to be straight), but masts really require very tall trees.

England's supremacy at sea in the Age of Sail came with deforestation of large parts of the country, so much that timber had to be procured in the colonies. One of the main tasks of the Royal Navy during the Napoleonic Wars was to keep the sea lanes into the Baltic open, where many of the indispensable raw materials were procured.

Conversely, USS Constitution's ability to shrug off British broadsides was due in part to her being built of extremely dense southern live oak (harvesting which required a different kind of raw materials, i.e., slaves - apparently conditions were so appalling that free workers would not stay on).

Your guild may have exclusive access to very specific necessary raw materials, like a carefully husbanded old forest on certain islands. Safeguarding this access from greedy interlopers would make for interesting plots.

Answer 11

Most of your people suffer from extreme agoraphobia.
Not just in the "afraid of open spaces" sense, which is the most obvious aspect of agoraphobia. Also the additional aspects of the condition: fear of getting outside of reach of other people that can help you if there is a problem and the fear of being in a place from which you can't reach safety.

This is not so much a problem on land. And the agoraphobia has become ingrained in the culture, so most people don't even realize they have this problem.

But being on a small vessel in the middle of the wide open sea... That hits all the wrong triggers.
A very short crossing to a nearby island might be doable, or coastal fishing within sight of land, but further out...

Some people will be naturally immune to the condition. They can be open sea sailors, but everyone else thinks they are either unspeakable brave or totally crazy (or both).

This will greatly limit sea-travel. Only if there is a strong need for sea-travel (like the shipping of essential trade goods) it will be viable to undertake a journey.
Real sailors will be wealthy men as their services are in high demand, but there is only a very limited number of people that can do the job.

Answer 12

No ocean going boats.

1600 km is a good distance. You would need a motor, or a sail. The guild makes the rounds and looks at boats. If there is a sail or a mast, or a rudder, the boat is confiscated and the owners are punished. Informers receive a bounty if they report the owner of a sailboat.

The only allowed boats are oar powered and rudderless and people use those to fish short distances offshore or to make trips around their own island. Oar power is not enough to cross the mediterranean.

The guild has sailboats and will charter trips for people who want to go distances. They will also intercept boats at sea in violation of their rules. Smugglers and rule breakers complete the start and finish of their trips at night, waiting out the days in camouflaged boats in the open ocean where they are less likely to be detected. On land, they have special disguised ports where they put in and hide their ships.

Answer 13

Space is inherently hostile -- we humans can't go out into it without full body protections from radiation and the lack of, well, much of anything that is out there. It's mindbogglingly large distances between solar systems that may or may not have anything worth while.

To bring that experience into a terrestrial world will require some translation as even if the boat sinks and we are stranded on the water, we can live for a while at least, enabling a chance at rescue.

New Specialized Knowledge

Sailing in this new and more dangerous world is not just a matter of wind and water. The magic in your world interacts with the world not necessarily in a hostile way, but in a harsh and uncaring way. Also not fully predictable.

Magical Knowledge

Navigation in this new world is ma mix of traditional navigation as well as some manner of magical-based skill set. It could be a specific set of spells kept secret by the guild that makes long-distance travel much easier (or even just possible). Given the lack of population, but higher technology, it could be that these long-distance mariners are a guild so that their specialized skills can be passed from master to apprentice.

That they control all long distance travel by doing this is a bonus feature.

A sense for the currents of magic are just as critical as the sense for the winds and the waves, and for how your vessel feels in the wide open. How to guide your vessel through the metaphysical currents of magic can and will save your life.

Of note, I'm trying to not specify a particular magical method of navigating as the type or style of magic was not mentioned, just that it was there.

Updated Mundane Knowledge

To borrow from The Square-Cube Law's answer, the new things to know about the high seas are kept a secret from the world at large to preserve their monopoly on the information. While everybody might know that the local kraken lives in Kraken Cove, few know that a mana current in the ocean between two islands hundreds of kilometers apart can wear a ship down to dust in minutes without the correct preparations.

Shipbuilding might now need some manner of magic rituals to help imbue the ship's hull with protection from the harsh world, not unlike radiation shielding that we will almost certainly need for long-distance space travel. The people that developed this desire strongly to keep this a secret.

General Knowledge

For short trips between close islands and for fishing, what we have or can cobble together from what was left after the apocalypse will work. If there is a peril near an island, you can be sure that the locals will learn of it and learn to work around it. They will have the technology to log it and to keep good records.

In essence, the islands and the bits of water around them are your solar system equivalents.

The Sky's The Limit

A minor frame challenge, but if you are not set on sailing in the waters, then how about the skies? If you are looking at 50's tech, then while jet planes and aircraft as we know them might not be a thing yet, airships definitely were something achievable. If I recall history correctly, the Hindenburg disaster spelled the end of an era for them in the travel department, and I know that was the 1930's

Like above, your guild could control travel by controlling how to create airships. The lifting gas may be different, or it might be some manner of secret spell or alchemy that they keep hidden to secure their aerial monopoly. Any of the sailing tropes and ideas can still apply to zeppelins, and now you have the real possibility of crashing into some very unfriendly waters.

Answer 14

tl;dr

A very weak magnetic field requires highly specialized navigational tools that can drive navigators into insanity. Some very few navigators (a guild) are able to withstand these side effects and navigate the oceans. This can even raise the question; if you end up in the wrong part of the world, was it because of weather, changing currents, bad luck, or is your navigator, although a member of the navigation guild, starting to lose his/her mind after all?

Some details:

A shift in the magnetic field so that it's near impossible to navigate by compasses. In order for compasses to work, you'll need to enable compass needles to move and indicate direction in a near friction-less device. Perhaps an orb the size of a fist in which magic or new technology cancels gravity. You could even build on the historic application of mercury in light-houses to create such a friction-less device.

In the 18th and 19th centuries, the lens was usually set on wheels or bearings and attached to clockworks, which the keeper would periodically wind. In the 1890s, some keepers began floating their lenses in liquid mercury. The lens’s metal base spun more easily in the mercury, which helped the light rotate faster with less frequent winding.

And also:

Modern scholars have wondered if mercury, not isolation, was behind reports of lighthouse keepers behaving erratically or losing sanity, since chronic mercury poisoning causes confusion, depression, and hallucinations.

What could make a guild in your world special, could be the ability to withstand the negative effects of operating, not light-house, but compasses, that needed mercury or another material / magic/ technology to function.

The change in gravitational field could also be linked to the rising ocean levels in your world.

Answer 15

The islands move and have no fixed position

If the island were capable of motions, it would be difficult to sail, since you have no certainty if and when you will be able to find a new harbour (or if after a pair of days you will be able to come b ack to the safety of your home port). Even more, it would also be very difficult to reach a particular destination.

You could add to this effect a lack of fish in the sea (maybe there is a lot of krill, but in the same event that flooded the world, fish became extinct), which would make traveling by ship even more risky, since the crew would be completely reliant on the scores of the ship for food (more food to load on the ship would mean less cargo and less fresh water).

Luckily thanks to magic, some gifted individuals can "sense" the presence of the islands (they're called Goal Perceiver Shamans, or GPS'), which means that every crew would need one of these navigators (certified by a guild) to lead them toward the islands. Also, there is a hierarchy among these GPS', since the most valued ones can sense the location of an island from farther, or can figure the location of a specific island (rather than any island).

For the reason why the islands would move:

• Well, magic (which is the reason why GPS' can sense their locations)
• They're basically floating platforms and move along the sea current, maybe a kind of natural conformation or a particular kind of coral reef; Sea current are very irregular, so it is difficult to predict the position of an island in the future
• Giant turtles (which would make their path more erratic than a simple floating island, whose path along sea currents could somehow be predicted)
• They were huge platform built during the flood event to save the most people, and their origin has been forgotten since then. They are engine-moved (powered by still not exausted nuclear reactors) which the reason why they move in an unpredictable way; in addiction, the background radiation of the engines is what in reality allows GPS' to perceive the presence of an island

Answer 16

Elvers. The interstellar contacts of your radio operators and mages brought the attention of the Great Eels, who blasted away from their previous planet on columns of nuclear fire. They formed a great Ball hundreds of miles across to weather the crossing of space. They dispersed and rained onto every part of your planet's ocean, which they sensed from space, for forty days and forty nights, until the entire level of the sea was raised.

A very few, the Supreme Eunuchs of their race, remained alive to watch for danger and to map out even the deepest of your planet's uranium deposits. The remainder dissolved into gametes, giving their lives to the process of natural selection.

The larvae or elvers of the Great Eels receive all the non-nuclear nutrients they need from the sea and one another, and produce energy by nuclear fission. (With a little magic, induced gamma emission definitely will work, even at the smallest scale). Their task for the coming centuries is to ingest every particle of digestible isotope on the planet. But their lives are constant struggle, an eternal facing off of contender against contender. The losers feed the winners until one day the great cycle continues once more. When they tap into their stored resources and blast aloft, the nuclear fallout will utterly sterilize the planet of all other life.

Until that time, your people are free to do as they wish. The Eels remain in the sea - they nearly are the sea - and will not bother them. Unless provoked, that is. When dealing with outside forces, for Elvers fight and flight are all the same: they activate their immature space drive and fly away on what ranges from a geyser of contaminated water to a pillar of flaming fallout.

Because tampering with the Sea of Nightmares can destroy everything within hundreds of meters and spread fallout for hundreds of miles, humans are wary of it. There are few buildings nearby, those tightly regulated. The sea is crossed only by a guild of Navigators, people chosen as toddlers because they naturally play well with animals. Their entry examination is to play on the hole of the asp and put their hand on the cockatrice's den. A skilled Navigator can not merely launch a boat without being destroyed, or avoid being overturned and slain on the Sea. Navigators can understand and empathize deeply with the vast horde of Great Eel larvae all around them, and by subtle hints and motions, place his ship in a path to be moved forward toward the destination. There are tales of ships that made a passage at a hundred knots ... but far more of ships that never returned.

Answer 17

Determining longitude and latitude to properly navigate.

Longitude is the vertical lines on a map. Latitude are the horizontal lines. By calculating each line you find your location at the where they cross.

They were able to calculate latitude via the angle of the sun at noon. As the sun in different parts of globe has a maximum angle it. Thus if you were at the equator the sun would be directly overhead. There's a chart of the angles and by taking the angle can find the latitude lines you are between.

Longitude required keeping time. Greenwich, England was used as the starting point. There's the Greenwich Meridian which represents 0 longitude. Then you set you clock "chronometer" to Greenwich time. Now when the clock hits noon you know the sun's angle at Greenwich. Thus, measuring the sun's angle at your location allows you find the longitude line you are on.

The Chronometer was difficult to develop because due to the rough conditions of the boat (movement and water) clocks were not very accurate. Finally in the mid 1700s they developed a reliable chronometer. Before this navigation was extremely dangerous. In fact due to a disaster the English government offered a reward for the person who invented a reliable longitude navigational aid.

https://en.wikipedia.org/wiki/John_Harrison

The guild could have access to a chronometer before anyone else allowing them to navigate accurately from trade port to trade port while others flounder or get loss.

Answer 18

Some thoughts on how to make 'acid' or poison water work

Just like real earth has fresh water and salt water, your world has 'poison water' and 'safe water'. People on land don't have to worry too much about it so long as they stay clear of the beaches. Rivers, streams, creeks generally run safe water into the ocean instead of the other way around

The poison elements in the ocean do not persist in the air very well. Nitrogen (N2) molecules will neutralize it. But if there is a strong wind from the ocean in the direction of land, it will be carried over on 'larger' (i.e. more than 6 micrometers) water droplets, and so people within a few km of the coast will be in danger. They will need to hunker down in similar ways we hunker down during a tornado, hence there are a lot of wind direction + wind speed meters near coasts, but no communities except operation plants. However, the poison gets neutralized fairly fast when in the air, and so anything more than 15 to 20 km landward people would be safe without need for protection

That said, direct exposure in the ocean can be highly dangerous, especially over prlonged times (more than an hour). This chemical compound reacts very fast with nitrogen (and gets neutralized as a result), and it is corrosive to carbon-based materials such as wood and plastic.

The byproduct of this poison chemical with nitrogen results in another compound that contains nitrogen atoms in the molecules. However, medium temperatures (e.g. 15 Celsius or more) + sunlight will restore the nitrogen molecules into the atmosphere while the atoms from the poison compound will end up in another form that is not poisonous. Hence, although this ultimately reduces the amount of N2 in the atmosphere, the quantity is relatively stable

As a result of this is there are large corporations that specialize in transporting people to and across the ocean. You can also conceptualize other factors, such as politics and federal regulations requiring safety standards for any individuals to cross the ocean, i.e. anyone wishing to cross the ocean must demonstrate to the authorities that they have met certain safety criteria, by which they get a license to travel the ocean, and for which there is a fee (of course). This license must be renewed quite often, and because of corruption the price is fairly high, which prevents the average middle class person from doing so without considerable thought or necessity

But certainly, if you wish to take this approach, see if you can pass your ideas by chemists and environmentalists to see if they have thoughts about it.

Good luck and have fun with the novel. I'd love to see it when its done

Arts, crafts and sciences uplift the world of being and are conducive to its exaltation ~Baha'u'llah

Answer 19

Sailors use natural ocean currents and wind patterns to power their travels. In your world of relatively small inter-connected seas, each "sea" has its own currents but those currents are not oriented in a particularly useful way. Perhaps they're circular (like some of the Mediterranean currents), or they dump your ships into open ocean instead of sending them towards other islands. The return path of a sea current might occur at depth instead of on the surface, so trips could become one-way. Combine this with doldrums that make sails almost useless and you have a situation where sailing is extremely difficult.

But not to worry, your Guild of Open-Ocean Navigators are here to help! Hire a local GOON to handle all of your transportation needs. Their mechanically-motorized vessels can propel themselves, without wind and against the strongest ocean currents. No more risk of being stranded until starvation or swept into enemy territory. GOONs can take you directly from point A to point B, efficiently and safely. Accept no substitutes or imitations! Only licensed GOONs have access to the fuel that powers these marvelous machines.

Answer 20

Syrup

Whatever caused sea levels to rise also slightly raised the viscosity of the seawater, so that more work (whether provided by wind, humans rowing, or a motor turning a propeller) is required to travel the same distance. From an economic perspective, distances between islands increased more than just what the geographic distances would suggest.

Changing the viscosity of water would probably have disastrous effects on most things living in it, but perhaps living things can generate a magic field that help them move around enough to cope with it anyway — or conversely the increased viscosity is a magic effect that shuts down around living things (in proportion to the size of the organism, so that the plankton won't just disable the effect). The viscosity of water inside living cells must not be changed.

Possibly this sort of thing would lead to a shift in hull designs, towards catamarans and maybe even hydrofoils, but I'm not qualified to comment upon that.

Answer 21

Some suggestions that I haven't really seen other people bring up...

Large, territorial megafauna

Humans are kind of lucky in that most of the extant marine megafauna aren't huge, and of those that are the majority are non-aggressive filter feeders. Of the large predators that do inhabit the oceans and pose a threat to ships, orcas are weird in that they seem to deliberately avoid antagonizing humans, whereas great white sharks can sink small canoes but cannot harm larger ships.

Sperm whales are about the only large ocean animals that are known to attack ships. They don't deliberately do so, but they have been known to sink ships several times their size if wounded by whalers or if they believe the ships to be intruding on their territory during the mating season. An ocean filled with larger predatory marine life that can either view a ship as prey or is highly territorial and has some mechanism that can deal damage to a ship (like the sperm whale's spermaceti-filled battering ram melon) would make sailing much more hazardous because now there is ocean life that can stop a voyage in its tracks.

If ocean life is aggressive enough it may even slow usage of smaller boats like canoes, if moderately large predators are attacking and sinking small craft every time they get in the ocean. As a result, even if larger ships would be save humanity might never take the necessary intermediate steps to get there.

Some species may choose to travel to certain locations to breed or raise young at certain points of the year, much like whales and great white sharks do today and megalodon is thought to have done so in the ancient past. If sufficiently territorial, this would make some islands virtually impossible to travel to for some periods of the year, lest the ships be sunk by angry males believing the ships to be challenging them or females protecting their young. This would force these islands to be highly self-sufficient or starve given they would be frequently cut off.

Extreme tides

Extreme tides, which would have to be caused by some kind of shift in Earth's rotation or Earth's relationship with the moon, could potentially make sea travel extremely hazardous. What this would do is cause large areas of the coastline to be underwater at high tide, but exposed at low tide. This would result in boats very easily becoming stranded at low tide if they anchored too close to the island, or potentially wash rowboats out to sea if people are not very, very careful where they anchor them and drag them far above the high tide line.

Another consequence is it would make navigating nautical hazards very unpredictable and unreliable. An outlet that may be perfectly navigable at high tide could be filled with dangerous reefs that would pierce a hull at lower tides. What areas could and could not safely hold a boat would be extremely variable and depend on detailed knowledge of local topography.

Rarity of Fresh Water

Abundance of fresh water typically makes or breaks human habitation of islands. Of the few islands that were not successfully colonized by pre-industrial/age of sail humanity, most of them, such as Aldabra or the Galapagos Islands, lacked any permanent sources of fresh water, meaning they relied on imports of water from elsewhere to be habitable and could not be permanently settled without being supplied water externally. In a world where most of the landmasses are flooded, it stands to reason that there would be a lot of areas that lack fresh water due to the small size of most landmasses. This would mean that ships would have to bring all their water with them or risk death by thirst, making travel in these areas extremely expensive.

Most islands without fresh water do have large tortoises that could be used as food, but this is mostly because they lack fresh water and therefore humans. Tortoises are known from most large oceanic islands but the ones that had fresh water on them were all wiped out shortly after human arrival (e.g., Madagascar, the Mascarenes, the Bahamas, Bermuda, the Antilles, the Canary Islands, etc.)

Extreme doldrums

Other users have already mentioned this, but I would add in a world with few major landmasses, there would be no continents to disrupt the flow of ocean currents and as a result crossing the equator would be very difficult. The doldrums would stretch virtually unimpeded around the globe and there would be very few ways around them. Few currents would travel through this zone because there would be no trans-equatorial continents, and people would be unable to hug the coastline or travel on foot across the continents to avoid them.

It may be harder to monopolize sea travel than you think

It's amazing to think how far humans are able to get with so little technology. Humans were able to disperse from Indonesia to Australia/New Guinea over water 50,000 years ago, with otherwise Stone Age technology. Furthermore, the Polynesians were able to disperse to nearly every island in the South Pacific based on a somewhat limited tech base. Polynesian sailing was very refined and the Polynesians had a complex knowledge of shipbuilding (using designs like outrigger canoes) and navigation, but the Polynesians made their actual boats without the use of metallurgy or heavy industry (which if anything only makes the feat more impressive).

What this goes to show is that with enough time and motivation even Stone Age peoples would likely learn to be able to navigate the ocean, especially if the world was a series of islands and ocean travel was critical for survival. If people didn't learn to sail it would make them incredibly vulnerable to being wiped out if a volcano erupted, the environment was depleted through hunting or deforestation, or if the environment became too heavily degraded for farming or fishing the local waters. Even if the seas were hard to navigate people would try anyway.