Love the tiles and idea, especially the colors. For actual map design though, I think there's gonna need to be a bit less depth though, since Link wouldn't be able to just shoot up over the layers of wall, right? I mean, this is still a 2D game so we should consider how the mechanics should work. I imagine it's gonna be like Oracle of Ages' mermaid suit or HM Dive in Pokemon sapphire/ruby, where it's basically like walking on another map, but with a different animation (swimming).
Or are we gonna have great changes in undersea elevation, but Link stays swimming at a certain elevation near the surface? So you'd see the floor drop out, see different heights below him, but he'd just swim right over. If so, we could wall in the maps with certain walls, while other walls he just ignores since he's actually near the surface you can't see. If this is the case, how will he interact with things on the seabed?
Lol, basically, same question about showing Link's relative depth as Miles, just he probably phrased it better.