this is a pretty cool underwater exploration game, I don't think I've played anything like it before, but after a while the game gets a bit repetitive because of the limited amount of upgrades, I have some suggestions on what you could add to this game.
1: add the ability to be able to swim up to the surface if the character goes up high enough.
2: add some things that can attack you, like sharks for example, when attacked you will lose some oxygen which will make it more tense trying to go back to the sub.
3: add some new upgrades like a spear gun if you plan to add any hostile creatures.
4: make the map bigger.
so those were my suggestions on what I think could make this game much better, I hope you see this comment.