The UI and art style are incredibly good although the gameplay feels a bit too random due to the lack of player control.
The oxygen mechanic seemed only purposeful in the late game as there is (as far as I can tell) no incentive to stay around longer except to get to the mermaid (could be remedied by needing to return to submarine to collect fish).
Early game bubble fish are really neat since they seem to spawn behind the player and the tank controls make getting the extra oxygen a significant time investment which is a neat form of player expressions.
Would be nice if the background had parallax because it's a bit hard to tell if/where I'm moving.
Ending was a bit of a bummer but also quite funny.