The thing I feel is missing the most is waterweight. Currently all movement feels like it meets no resistance.
I kinda would like it more if it was full 3D but I believe you can also make 2.5D work.
Cool! I think it would be nice to make the sub feel more like it's moving in water and experiencing more realistic physics, even if still simplified. Should I do much more, I'll definitely aim to achieve something like this.
And thanks for your vote of confidence on the 2.5D! (: I do think it could work well in a final game.