Brilliant. Very fun!
My main problem is the lack of fullscreen mode. I had to battle the web browser scrolling too much!
I think the idea of a gun separating the waves could be explored more and be a very interesting feature. You've got two games to explore here; a 2d defend the beach shooter, and a game where you separate waves to do something.
Wonderful game though! Check out my game and see if it's just as engaging! :)