Hey!
First of all, congrats for making the game! I love the visuals and the gameplay was fun. It was pretty unique I would say.
If you want some feedback, here's mine: I played until -2600 when I found that my submarine rotates after my mouse. No tutorial or anything else told me that, so it was a little strange to find a crucial info that late. I saw that you took care to explain how to upgrade/downgrade so that's good. Another thing is the map, I don't know if it's generated but I was no able to find any opening big enough to go deeper. So it was impossible for me to progress. The submarine was also really big so it was hard to move it around, it would be more fun to be half of the size, at least for me.
But those are things that I'm sure can be easily fixed. Nonetheless, a great game with solid gameplay and amazing visuals. You did very well!
I just rated your game, maybe you can do the same on mine ^_^