I've been messing around with the concept of an underwater world for a story, but I've run into some problems figuring out exactly how it would work and what it would look like on a map. I've thought about undersea cave/caverns to cities built with coral (think Disney's The Little Mermaid) to sunken ruins. But nothing strikes me as inventive or incredibly plausible. Anybody got any other ideas? I'm already making the assumption that everybody can breathe underwater and swim so pockets of breathable air aren't necessary.

At this point I'm thinking of a cross between all three. Natural underwater catacombs beneath an island (volcano or earthquake related perhaps?) surrounded by warm, shallow waters filled with coral reefs. Ships see the island, come to investigate, and crash on the reefs (graveyard of ships anyone?). That would have diverse fish and plant life while leaving room for above-ground access for story/role-play potential.

I'm assuming I can find some nice tutorials/walkthroughs for the catacombs part, but how does one go about mapping a city made out of coral and shipwrecks? I don't think it would be a big deal except I can't free-hand draw to save myself (no tablet if I could). Any ideas?