I love this idea!! I would be concerned about having latex shoved up my hoo haw for any length of time though. Also I would use a plain white or clear balloon so you don't have the dyes leaching into the skin as well as the latex.

If I knew it would be safe then heck yeah I would try it. What a feeling that must be to have the weight of the water sloshing around. I would want it to be very hot since I don't like cold at all.