I really loved this one. A lot of fun, but there are some things I'd tweak:
I wish I could hold down click to shoot.
I think you should be notified of a new wave before you get your upgrade, I accidentally clicked on many upgrades.
I feel the oxygen bar should be blue or the stars green, because my dumbass didn't connect the two until much later
The idea of the risk reward with the oxygen for vertical movement is really cool, but you could basically just avoid it by staying on the ground and the oxygen cost was small enough that it never really was a limiting factor. I feel like maybe make the jumping more essential or tie oxygen to other movement or shooting or have it deplete faster.
The character and the fish are super cute, but the background felt like a bit of a hodgepodge imo
But I had a really good time with this one.