There's some nice sound effects. I like the light on the submarine. Maybe you could even make the contrast between lit and unlit areas biggger to sell the deep and dark effect. The movement could be tweaked.
For a game like this you need to have snappy controls but currently they're slippery. I would probably remove the gravity, increase the acceleration and make a speed cap. In engine you could make the speed cap by checking if the velocity is bigger than the max and whether it would increase when you press a key. Then you wouldn't add a force.
But overall the idea is still nice and I really enjoyed some of the levels.