Cute game. It's a bit linear, I'd love to see the maps restructured so that the beginning areas have places you can't get to until you've upgraded your sub.
Also, to be really nitpicky (I'm a diver), you don't breathe oxygen underwater, it would kill you. Divers and subs use air tanks filled with 21% oxygen and 79% nitrogen, just like the normal air we breathe.
 "Deep Diver"){kind=icon}
 "Critter Forge"){kind=icon}