Even outside of water, Namor's powers are extensive -- and JB even removed his power of flight early into the run (can't remember why, though I'm sure it's in the FAQ somewhere).

I think water, like outer space, is a tough sell since we humans don't live in water and don't live in outer space. It's an alien environment that doesn't adhere to physical laws humans are generally familiar with. It's not hard to figure out why readers can't clue in to water adventures as much.

What works is an undersea compound, or a floating compound like Hydrobase. In the undersea version, the threat for non-aquatic characters to die from hull breaches or the like lends itself to the story. The ocean itself is an antagonist.

One way or another, the aquatic hero who needs water sustenance can go the Triton route, with the danger being that any severe damage to the sustaining device could mean a suffocating death for the hero.

In general, I think readers can understand the dangers of the ocean more than the life-preserving ocean for a non-human. So when doing Aquaman, I think you keep him dealing with surface folks who are vulnerable to what he is not vulnerable to. In that, he becomes their Superman. Out of the water, Aquaman can need help both in acclimating to surface culture and the inherent vulnerabilities of an Atlantean/human.

I think for my money that Abe Sapien in the Hellboyverse is probably the most successful aquatic hero I can think of. I'm not even sure Abe needs water sustenance in the comics, though the movie version has a kind of scuba suit to walk around on the surface with.

Abe rarely is seen beneath the waves, but when he does, his experience is usually mirrored by human anxiety. He's obviously an ocean dweller, but just as obviously he only has a limited view of the totality of the ocean. There are many, many things there Abe has never experienced, which connects the reader to him emotionally.