I think I'm one whose body doesn't absorb enough water. I guess there's a fairly new medical theory about it (read about here on this site): some people's bodies send too much of the water they drink directly to the bladder so you get to struggle through life with a dry intestinal tract and therefore constipation. It causes other problems as well, you're basically dehydrated so matter how much water you drink.

Last I heard, no one has come up with a cause for it yet; if I had to guess, I'd say it's a problem with the "fight or flight" process in the body. What works for me, only once in a while (but better than nothing, I guess) is to drink very warm, almost hot water very quickly. Supposedly, that activates something in the brain saying "this is important, so process this water correctly this time." The water has to be both very warm and drunk very, very quickly, though or it won't work.