I liked the take on controlling two objects at once, and where in many of the other games it would take me a while to get to terms (or never get to terms) with controlling two objects, in your game it felt very natural right from the start.
I did have a little trouble making use of the cannon, and I think this was a combination of it being a parabolic arc and altering where it was aimed as I moved forward and backwards (or maybe I just didn't ever understand how to aim it) so I ended up relying entirely on the sub's torpedoes for damage.
I got confused as to how to eliminate the barrage of incoming fire that appears from below the map - and that ended up killing my sub and ending all my attempts at progress.