It's incredible, man. When I first played, I was pretty distracted w/ my own jam. Now that I've gotten a chance to dig my teeth into it, there's so much to love. The progression feels so satisfying, and hurling up to the surface with just a drop of Oxygen left feels exhilarating like Subnautica. Very cool that you built it all yourself, but I'd expect nothing less. I couldn't find a single bug, so nice attention to detail.
On the constructive side, I think a gradient background would go a long way to make the depth real to players, the gems could be a different color to better differentiate as a pickup. Mechanically, I thought it could be more fun with even a little more default oxygen still. I noticed that the shop items are nice and cheap, but I assumed I'd have to save up more at first, so maybe just some guides for the user like "You have enough to go buy something at the shop, go do that dummy"
Trying out the pipes now, I get it, I actually like this mechanic much better in 2D than Subnautica's pipe system. Fantastic stuff.