Not a bad entry for your first jam! I think the concept is solid, but there are definitely some small adjustments that could go a long way towards making it more fun. The biggest issue I ran into was that the gold was not being counted correctly, and most of the time it took away from my gold count, making it impossible for me to upgrade anything after a certain point lol. The movement wasn't bad, but I found myself constantly getting stuck on edges of the environment; I think if the tunnels and pathways were twice as wide and tall it would make it easier to move, but the challenge of figuring out where to go would still remain. My only other suggestion would be to make the oxygen and gold count static elements on the screen somewhere, as it can be challenging to read while swimming and moving around in different directions. Hopefully you can iron out that bug and make some adjustments, because I still think the core of the game is good!
You can watch my gameplay video playtest below for some more feedback: