I didn't really feel threatened by anything until the sharks arrived and that was quite a while. Maybe you could either add some fish incoming from the left or some coming from either the top or bottom of the screen. That could diversify the gameplay.
I would like it if you would always shoot to the right, no matter where you're facing. You don't really get a lot of fish to the left of you and when I tried to the left it kinda took me from the flow of the game. Also the shoot animation only plays when holding space for a prelonged period of time. If you just press space you don't actually get this animation.
The bubble sound effect started being a little irritating. Instead I would recommend adding some royalty free music. I did however like the choice of colors.
Good job!