This is so much fun! I wish the difficulty was tuned to be a little easier though. For example, on oxygen depleted, if instead of instant failure it started a brief high-pressure countdown so you can "hold your breath" and return to the surface. The core gameplay loop totally works though, and gives me the urge to explore and collect. I see from your comments you had balancing in mind but didn't get to it, and that's really my only critique. I'm going to play a little more and see how it feels after upgrades.
Visually, I loved the bubble animations and the rolling waves on the surface. The dithered wipe transitions are unique. High level of UI polish, super nice to have a save system too, which I see persists between page refreshes. Really impressive that you got all this done in a couple weekends!