I'm thinking about colonies that are (eventually) self-sufficient. I agree that it's easier to have a small crew of people living at the bottom of the ocean because it's generally cheaper to get stuff down there. But a self-sufficient colony is pretty much impossible, I think.
Building structures that can withstand pressure, never fail catastrophically, and can be repaired is really hard. In contrast on Mars you only need a bit of dirt for radiation shielding and any atmosphere leaks can be repaired at a much more relaxed pace. People can walk around on Mars in simple suits, you can't leave your structures at the bottom of the ocean at all outside of a deep sea submarine. Building habitats on Mars is essentially just mining, and we have centuries of experience with that.
Getting energy at the bottom of the ocean is also difficult. Nuclear and geothermal are the only options, whereas on Mars you can use solar panels.
Self sufficient on the bottom of the ocean is IMO no more difficult than self sufficient on Mars, _particularly_ when you consider how much infrastructure you could establish there with even a fraction of the cost of bootstrapping a Mars colony.
Building structures that can withstand pressure, never fail catastrophically, and can be repaired is really hard. In contrast on Mars you only need a bit of dirt for radiation shielding and any atmosphere leaks can be repaired at a much more relaxed pace. People can walk around on Mars in simple suits, you can't leave your structures at the bottom of the ocean at all outside of a deep sea submarine. Building habitats on Mars is essentially just mining, and we have centuries of experience with that.
Getting energy at the bottom of the ocean is also difficult. Nuclear and geothermal are the only options, whereas on Mars you can use solar panels.