The Master Speaks:
http://www.straightdope.com/columns/read/5/how-deep-is-whale-shit
A truly classic line.
Regarding whether you could poop underwater, even without what has been said above, it seems clear that you could. Catastalsis is responsible for moving feces through the intestine, and if peristalsis can allow you to swallow a banana while you’re hanging upside-down, the intestinal equivalent ought to allow you to excrete while preventing “backwash” or enema-by-water pressure.