In a very real sense, what makes the aquatic hero different is also what threatens to alienate him from his audience: He has a whole, alien world to protect. On the one hand, the undersea kingdom, whether played as scientific fact or fantasy landscape, is a visually rich environment that's any great artist's dream - unusual and ample flora and fauna, the way the currents interact with hair and cloth, even the "surface world" of swaying waves, dangerous icebergs and tropical islands. On the other hand, readers are far more used to their superheroes fighting the good fight in urban areas, and can relate to the Big City even if they live in the country. Buildings, cars, fire hydrants, the threat of a mugging, these are familiar things the reader can latch onto, useful aids in the suspension of disbelief. I can believe a man can fly because I can believe these other elements of the world. The world of Atlantis, however, has none of those recognizable touchstones. We can't even take something like the difference between night and day for granted in the undersea world, not at the depths often shown. And surely they have a different culture down there. So while we can oooh and ahhh at the wonderful world created by the artist (and you can see how quickly even that feeling can leave us under a lesser pencil), we can often feel removed from it, disconnected and disaffected. This isn't that different from the hero who patrols outer space (like Green Lantern) or an alien world (like Adam Strange) or a mystical dimension (like Dr. Strange), but when those characters have left Earth behind for good, they tend to flounder as well.
And yet, though we may leave for safer shores, we will return to the mysteries to the sea again. And when a brilliant writer is teamed with a brilliant artist, both willing to embrace the aquatic hero for what makes him unique, Siren-like, it lures us back. World-building takes considerable talent, and that's often what's required. The temptation to dedicate the hero entirely to environmental and conservation concerns is a potential trap that can make the book preachy and readers may resent the hero. Obviously, pollution and over-fishing are concerns in today's oceans, and the hero should be confronted with them, but it shouldn't be his or her single focus, no more than an urban hero would obsess over a single type of crime. Because landbound heroes aren't doing a whole lof of environmental activism even though there's a lot of pollution on the surface world, treating the aquatic hero differently only creates another layer of distance between the reader and his or her expectations. And since the surface world is your main polluter of undersea environments, it will be make the hero clash with the reader's own world. The Silver Age Aquaman stories, perhaps the golden age of aquatic heroes, had him policing the seas FOR the surface world, a friend in a harsh, extreme environment. There's a lesson in that, surely.
What attracts YOU to aquatic heroes?