Good game; I liked drawing the bubbles, but it was too easy.
For the mines, subs, and red barrels you can just draw a small bubble in it; the bubble will hold whatever you drew it in, and shake it until it explodes. Same with divers and shrinking bubbles, but it instantly kills them. There isn't much of a reason to draw big bubbles that use up all your air if you just use those two bubbles to kill everything.
If there is ever a sequel or something, make it so that bubbles don't hold things unless they are bigger then what is in them. Other than that, it is great.