What are the advantages and disadvantages an underwater civilization has over a terrestial one?

Question

In my world, mankind isn't the only intelligent species around. Merfolk are present in all of the oceans. The Merfolks look like a cross between humans and dugongs. The Merfolk also have long hair they use as gills.

The Merfolk start a civilization around the same time humans start theirs. What are the advantages and disadvantages the Merfolk will have over the humans? Here is what I have come up with:

Advantages

• The ocean on Earth is far larger than the land on Earth. This would mean that the Merfolk will have access to more territory and resources but they will also be able to support a larger population.
• The ocean is interconnected, you can travel to any part of the ocean without having to fly or move over land first. You however cannot travel from the USA to France without getting on a boat or a plane first.
• The ocean is more uniform. Ocean biomes are less diverse than land biomes. A land civilization near the arctic is much different than a civilization in the tropics. Oceans are less diverse however so a successful civilization in one part of the ocean can copy its structure to many other parts of the ocean.
• The ocean is far less affected by bad weather and natural disasters. Floods, lightning storms, tornadoes, hurricanes, avalanches, droughts, and even tsunamis can be mostly avoided by just diving a few meters underwater. This will make ocean civilizations more stable over long periods of time.
• The aquatic civilization will be able to domesticate many unique underwater species, although humans will also have domesticated flora and fauna.
• Living in three-dimensions will allow for a lot more options for architecture compared to buildings on land.
• Sonar and sound-based communication is more effective underwater than on land.

Disadvantages

• Harnessing fire and therefore forging metals is much harder underwater than in air. There are still ways for the merfolk to do it like building forges on ships and using geothermal vents and trading with humans but overall, Merfolk are going to be far weaker in the metallurgy department.
• A lot of building and crafting materials that work fine on the surface don't really work underwater.
• Making light sources is harder underwater than in air.
• A lot of ranged weapons and tools humans use will be a lot less efficient, although this might not matter to Merfolk if they mainly deal with other Merfolk.
• Depth charges and explosions are far more effective against Merfolk than against humans. This won't make a big difference in Merfolk-Merfolk conflicts.
• Water is heavy compared to air. As a result, making a supermarine or a vehicle for Merfolk that explores the service is much harder than making a submarine for humans due to all of the additional weight. Seaplanes for Merfolk will also be far less efficient than Seaplanes for humans.
• Space travel is harder for Merfolk because not only is it hard to launch a rocket underwater, but you have the weight problem again of water being heavier than air. Once Merfolk due make it to space however, they are already used to navigating a 3D environment.

So given the advantages and disadvantages and more, would an underwater civilization progress faster or slower than a land civilization?

Answer 1

Your Merfolk are way more disadvantaged than you think

Fire is not just for smelting iron, but it's used for making ceramics, adhesives, cement, plastics, glass, etc. Also, hydrothermal vents cant actually be used for metal working because they cant achieve carburization; so, even if they could forge iron, it would not be any tougher than lead due to the aquatic environment. Even if the Merfolk could trade with humans for higher quality metal tools, these human tools do not survive long in the salty waters of the oceans. Iron tools would rust away in weeks. Wooden handle will become waterlogged and fall apart in your hands. Even if your Merfolk had full knowledge of every kind of human technology that exists today, they would still be stuck in the early stone age because almost none of it can be made under water.

Your assumption that they have more stable, livable area is also rather false. The vast majority of the Ocean is actually quite barren. Most macroscopic life exists only on the continental shelves where the sun can reach the bottom. This amounts to about 27 million km^2 of habitable land as opposed to about 64-100 million km^2 of habitable land available to humans (depending on what you consider habitable). Continental shelves are also prone to powerful riptides meaning any construction benefits your merfolk get from buoyance would be offset by "whether" that is much more destructive than wind.

Because deep seas are mostly barren, that means that crossing them would be as big of a deal to your merfolk as the Sahara Desert is to humans. Sure, it is "interconnected" but it will still represent a significant barrier in moving around.

Also, you don't get as much 3d space as you think because most aquatic life is evolved to live within a relatively narrow band of depths thanks to how quickly water pressures changes as you go up and down. Also aquatic life tends to be much more sensitive to environmental changes so while the oceans tend to be more stable, small changes in the ocean are known to cause massive die-offs.

Even Sonar is not really an advantage. While it's better under water than no sonar, it lacks resolution. The distance and clarity you transmit signals through the air is much better than what you can do under water.

Keep in mind that here on Earth we do have aquatic animals like dolphins and cephalopods that seem to have intelligence similar to humans, but because they are aquatics, they are stuck with only very limited tool use. Chances are your humans will not even recognize your merfolk as an intelligent species without a lot of time spent studying them.