The game is a lot of fun, the artstyle is interesting. I also like the sonar as a cool detail.
There isn't much to improve. I would perfect the system for losing treasures on collision and maybe tone down oxygen loss on impact since it can feel annoying from time to time (not too often). It would also be nice if it showed you some warning when the oxygen is low.
Overall, the game is great, polished and again a lot of fun.