Nice solid game which picks up in intensity as you level up. The music is absolutely beautiful! I wonder if you're willing to make the soundtrack available for download, that would be really cool.
The graphics I thought were a little bit hard to make out, and it would've been better to have some kind of explanation as to what the weapons are. Also since the game is supposed to be underwater, maybe it could be an idea to make player motion more swim-like instead of walk-like if you get what I mean.