I am fasting for Ramadan, which means for the entire month of August there is no drinking or eating from sunup to sundown.. so not even a drop of water as long as the sun is out. Women are not allowed to fast while menstruating though, so for 5 days I did not fast, but otherwise this whole month I have not ate or drank anything during the day. I seem to do fine as far as hunger goes, as long as I eat enough fat during the night, I don't much get hungry throughout the day. The worst part is dehydration, but that's probably only because I work at domino's delivering pizza in a car with no a/c in the middle of August Summer in Southern California.. so I'm pretty much sweating the entire time I am at work, and I can get pretty dehydrated that way.
Otherwise I don't see much benefit from the fasting from sun up to sun down. I have fasted before where I consume nothing but water for a day or two, and sometimes I feel awesome and better than ever, and othertimes I feel weak and tired and hungry. The energetic feeling is certainly worth it, but the tired and weak feeling is not.