I enjoyed how the movement controls were forward/back turn left/right. I enjoy when sailing or fish games do that, it helps to make them feel unique and drive home the point of being at sea. The ammo regeneration was a nice touch, the graphics looked good as well!
If you wanted to improve, I suggest adding more game 'juice' when you kill an enemy. The sound effect should be a little louder and more dramatic, maybe some particle fx too. The music transitions can be kinda sudden, I suggest a fade in and fade out effect.
Overall a fun game!