Cool game! I like the arts and the style.
Like the concept that going deep down into the sea, the deeper it goes harder it gets, It's basically the very concept I want to use in my game if I have time to actually implement it.
I think you should add some goals on the deep sea or add some more mechanics like letting the sub able to dash to avoid missiles in the deep sea, but hey if I keep telling you what to do it will just be my game but better (X
Anyway, you do a very good job and I like it!
BTW, found a bug that spawn you in the ground:
