Don't bother. They have no flash. A flash is an absolute necessity for underwater shots (if you want to see the real colors, that is).
I used one on vacation a few years ago. Aside from the scratched lens (a common problem in single-use cameras), the images were almost all "black and blue".
My suggestion? Go d*g*t*l. Canon makes some nice P&S models, all come with flash and there are 30m housings available for almost all of them at around $150. The photos you'll get will have real colors (you do need to be right on top of your subject - under 2 ft because of the weak built in flash) and there's no worry about running out of film.