Nice concept! The character/fish animations were neat, SFX was great, and the shop was a nice surprise, adds some replayability. The only thing gameplay-wise was that I found myself swimming in nothingness for a while to find enemies; it might be cool if fish spawned near you, adding more fish, or even make them notice and attack you earlier. I think it would be great to have a background with fish, plants, sea creatures... It would add lots of live to the game, and make the world feel much much bigger! Didn't find many bugs, only that the music didn't loop for me. Also, some impact sounds when shooting enemies would make a great difference in game-feel!
Game's great fun! With some fixes here and there, it would make for a great experience. Keep it up!