The background art is amazing! The character/fish/bubble sprites are good, too. Same with the music. The character moves fluidly and feels very responsive.
Criticisms:
The colliders are a bit odd, though. I would get killed by a piranha who was within an arm's length from me. General tip - you want the colliders to be a little bit smaller than the character sprites, otherwise it feels wrong to the player.
The upgrades were way too expensive, costing thousands of dollars. I got 30-200 dollars for each try, and they didn't stack either.
The piranhas didn't seem to have much of a survival instinct, most of them flew into spikes right as they appeared on the screen.
This is a pretty good game as it is though, wouldn't think it's your first. Well done!