Nice game! A diving bell that dives deeper into the abyss is perfect for the theme, in my opinion! I really liked the way movement works with the gun's recoil, tho I have to say it is a bit hard to control due to the phisics-based movement, and the player can get outside of the screen pretty quick if you shoot too much. The landscape feels a bit empty, I would love some indicator that you are going deeper, like some different tiles when you get to a certain depth, or some different enemies and hazards. The swordfish got me consistently low on life because it was too fast and I didn't see it, or while shooting it I got moved around and crashed on hazards or terrain.
In the end, a very simple but interesting and fun game with a great theme, it just needs some extra polishing, and maybe some added content. Keep up the good work! :D