How would you form an extended stretch of coastline that is effectively useless for overwater trade -- i.e. there are no sheltered areas to serve as ports, or trade routes that can reach all the way to the shore to allow ships to trade? It seems that a coast with cliffs would be an important element of this, but what would prevent paths up and down the cliff forming, or the cliff being divided into segments (say, by estuaries) for that matter?
You could have dangerous reefs, a tricky tide with heavy waves and strong currents due to the shape and size of the reefs, just far enough off the beach to prevent trade vessels from coming near enough for practical purposes.
That way, you can still have fishing boats between the shore and the reefs, you might even have some small-scale trade, where the cargo is transported over the reef in small boats, but nothing major.
- Have the coast along a divergent tectonic plate boundary and you can hope to have a very long steep coastline. See the East African rift or lake Baikal in Russia if you want to have an idea. It's not exactly as steep as the cliffs of Dover but it's still very difficult to put infrastructures on it.
- Have a strong uninterrupted wind, flowing all around the planet. Winds around Antarctica are very strong with an average speed around 30 to 40 kmph. Strong winds also tend to come with bigger waves. That's a big problem for traders.
- Funnelling the water and the winds into a narrower water body will also speed things up.
- The coast is steep but a lot of debris have fallen overtime, making navigation treacherous. Just having an unequal water dept with patches of shallow waters here and there could make navigation much more complicated. As an example, navigation on the St-Lawrence river requires captains specialized on that specific river because there is a lot of shallow areas, although it's not as windy as in my configuration.
- Having a large temperature differential will make the winds even stronger. But I'm not sure it's possible to do that with that kind of configuration. Having two large landmasses close to each other might not produce that effect. But I'm thinking maybe one of the landmass could be long but thin. Supposing that the large landmass is north of the passage, the thin one is on the southern side: have a large ocean south of these two. In winter (assuming it's in the southern hemisphere) the large continent become much colder than the ocean in the south but during summer, land is also much hotter than the water. This temperature differential will help having strong winds.
You could have a coast line with craggy mountains, numerous fiords and ice shelves of large floating rafts of ice frozen fast to the coast and extending out into the ocean. Like Ellesmere Island's northern coast, to provide an example, where it is virtually impossible to land a ship during the time of the year that the ice is present.
You could also have a coastline with long term volcanic activity, like Kilauea in Hawaii with its world famous lava flows into the ocean.
I'd look at all sorts of coastal islands. Sea stacks, for example. There are plenty of naturally occurring islands out there that are inaccessible by boat. Typically due to: cliffs, reefs, and the sea itself (rough seas).
Cliffs make it impractical to build infrastructure for shipping, but you are correct in that people could always build trails. So I don't think it's the cliff that are necessarily the key element. Just one of the barriers. The bigger barriers would be shallow sands (possibly fixed via dredging), rocks and reefs (possibly fixed via demolition), rough seas (possible fixed by building sea walls).
All surmountable, of course, but typically highly impractical. If you look at the west coast of North America, you'll find a lot of of the coast is not easily accessible by large ships. Most ports are in fact inland (up river or within large bays).
Cliffs are an easy answer, just make them incredibly steep descending into very deep water, with no harbor or sandbar for protection. Think of the Cliffs of Insanity from the Princess Bride. Even better make the cliffs slightly overhung to prevent path building.
But this is Worldbuilding, and we can do better. Instead of building a vertical barrier, let's build a horizontal one. Certain kinds of swamps are impassable, where there is just too much water for too little ground. Mangrove swamps are still passable by boat so that won't work. Grassland swamps like the Everglades won't work either because special boats can get through. Our swamp needs to have densely packed trees surrounded by deep, quicksand like mud. The dense trees prevent skimmer boats from working and the deep mud precludes trails or any kind of cargo capacity if someone did try to slog through.
Fill the sea with monsters!
Anything in the sea that threatens the lives of sailors or the destruction of cargo would discourage traders from landing. People don't trade if the benefits from doing so are outweighed by the risks.
- Unpredictable storms / big waves
- Scary monsters
- Bad chemicals or bacteria in the water
- Dangerous reefs or rocks
A seafloor consisting of many, many rocky spires that stick up to near sea level. Obviously there can't be so many that passage is actually impossible but there are enough that it's unduly hazardous, nobody would go there other than for a dire emergency. In the calmest of times it's passable to vehicle like airboats.
As for why it's like that--there's a volcanic field underneath. It's not actually eruptive, though, but rather you get lava oozes. Something about the situation results in pillar formation rather than the usual pillow lava. This only happens underwater, any lava that pokes out of the water responds to gravity more normally and falls down. Thus the pillars top out just below the bottom of wave troughs.
This is an ongoing process, even if you mapped every pillar out there and used GPS you couldn't trust you wouldn't run into a new one. The acoustics are terrible, trying to map the pillars with mine-hunting sonar is challenging and unless you have several propulsars on your ship you can't safely hold station while you are trying to figure out where it's safe to go.
Shallow sandbars, rocks, winds and tides going straight towards the rocks. Take a look at the San Francisco Potato Patch for an example of where you don't want ships to be.
Also, underwater methane hydrate vents are one plausible explanation for how the Bermuda Triangle got its reputation. Methane gas bubbles up, the density of the water changes, ships no longer have the buoyancy necessary to stay afloat.
Alternately, perhaps the entire coastline is on fire.
The Centralia mine fire in Pennsylvania has been burning for over fifty years, and has enough fuel to burn for a few centuries. It's basically an emtire coal mine that is on fire. Gasses from the underground fire seep up to the surface in concentrations sufficient to be a hazard to life. Flames occasionally can be seen on the surface. The coal mine is about 8 miles long.
So I envision a section of coastline with modest cliffs. An underground coal seam is on fire. There are a few small natural caverns in the cliff, so some seawater occasionally comes in contact with the fire. Not enough to put it out, though. The water hits the hot coals and immediately turns into scorching steam, which then vents to the surface. So you've got carbon monoxide gas, scorching steam, and occasional flames emitting from the ground.
If that's not enough, throw in a lagoon being heated by the fire, causing a chronic red algae bloom. Give the lagoon a few outlets running down the cliffs, carrying the algae with it. Now you've got a permanently smoking, scorching landscape with flames and with cliffs that look like they're weeping blood. I think that would say Stay Away to anyone.
Ice calving, also known as glacier calving or iceberg calving, is the breaking off of chunks of ice at the edge of a glacier ...
Calving of glaciers is often accompanied by a loud cracking or booming sound before blocks of ice up to 60 metres (200 ft) high break loose and crash into the water. The entry of the ice into the water causes large, and often hazardous waves. The waves formed in locations like Johns Hopkins Glacier can be so large that boats cannot approach closer than 3 kilometres (1.9 mi).
The above is from the Wikipedia entry on Ice Calving.