Solid foundations for something more! Very decent loop of dive, collect resources, sell and upgrade to do it again. Risking oxygen to get more loot fits the theme well.
A couple notes: There’s no feedback when you take damage from jellyfish. A noise, some knockback, or visual effect would really sell the fact that you got hurt. Plus maybe some visual elements like electricity around the jellyfish to tell you its dangerous. It’s very easy to miss a small number in the corner going down.
Lateral movement felt inverted when you were upside down. You could maybe modify it so that lateral movement isn’t a function of where you’re facing, but relative to the world? Or maybe there’s no need to be able to rotate your physics body at all, instead you could just rotate the sprite.
The UI could be less obtrusive. Maybe some transparency, maybe making it look more “diver”-y and thematic. It doesn’t need to be quite so big I think.
Music, more sound effects, ambience, menus, settings, and a tutorial/blurb on what you’re doing and how to do so in game (either via story-delivered dialogue or just as UI prompts) would all elevate this entry.