Very well polished. I could see myself spending an hour or two playing this. The balancing was really good, and it "introduces" mechanics well. It does get a little repetitive after a while, but overall really good.
I will say that it was a little difficult to keep track of the air percentage. The bar at the top of the screen helped with this problem, but the score had the same issue, where I would have to look away from the action to see what I had. The score not being easily visible is not as big of a problem as the air, and I can't really think of a way to fix these issues myself. I think adding a little +100 or whatever the score earned is in place of the fish when you defeat them could provide a little more dopamine hit for the player to keep them engaged more.