Really? Given the main function of the large intestine is absorption of water I would have thought it should be pretty effective, especially if you poke the tube a bit further past the rectum. Drinking water is just going to pass through your stomach into the intestines anyway.unfortunately (or fortunately depending on your point of view) putting fluids up the bottom is one of the least effective ways of rehydrating someone. it's just ahead of trying to absorb it through the skin.
Allegedly the same reason why alcohol up the arse gets you significantly drunker than the same volume orally.