I like the concept and the way your worked with it. But I don't think this game really blows me away. The main problem is the curve of the difficulty. You are way too squishy to deal with the water part, but once you get certain abilities you're like a psychopath with an infinite ammo shotgun and you slaughter the demons up to stage 6.
I think this could be an epic concept to put into an overworld and have you explore multiple waters in order to find treasures or bosses. Maybe you should overlook it again and think what you can do with it and maybe I'll be one of the many who is willing to buy this from sources like Steam then.
By the way, the abilities that make this game way to easy are the "Float" and "Reload"