Played it some. A couple points:
*Why the square screen/layout? I don't know many devices that use this resolution aspect? What's your target device(s)? I suggest using 16:9 or 16:10 aspect ratios. So that we can see the oncoming subs from
afar and take action accordingly.
*Needs a lot more polish. Shake and/or flash the screen when we hit or our wall is hit. The bubbles when we destroy enemy sub is good, but they could move upwards to the surface for example.
*Needs variation: Other types of enemies, maybe a slower moving enemy with more health, or an enemy that can move up and down (I didn't play that long, I don't know if such enemies arrive later, but if they do, they should arrive sooner)