This has a lot of promise!
Very on theme, and love that it starts out nearer the surface and sets context for the player, slowly introduces you to going deeper and then to enemies and so on. It might be interesting to start with some sort of understanding of how one might earn money and thereby kit out the sub for future missions etc. - potentially you could start with some money available to you which you use for a basic upgrade and then go on from there. I think that could help the player be invested from the get go.
The art and music is impressive given it was made entirely during the period of the jam! the music was quite immersive (pun intended) indeed.
All in all, I think it's a solid starting point and I'd love to hear from you if you make more levels and ship this one!
Do check out our submission Reverb. We'd love to know what you think: https://itch.io/jam/pjp-game-jam-2023-1/rate/2079583