Beautiful pixel art, as always.
The controls are a bit too responsive (especially the swim button), meaning that it doesn't really feel like you're underwater, and it's hard to make precise movements.
On the other hand, there are far too many opportunities to replenish your air, so you can pretty much ignore the hazards, and you'll still be fine (perhaps air and health should be two separate resources?).
Obviously this was made for a game jam, but with more time spent adding content and polishing the gameplay, it could be a really fun little game.