one fo the issues with water (the reason quite a few diets recommend drinking until you swell up like an oompaloompa) is that when you alter your intake you have more wastes (celluar) to purge from your body, increasing water intake helps. it also serves as source for O2 for metabolism. that being said, what ive heard from the group that im in (medically controlled diet for rapid loss) is that you want 2-4 liters/day, they are concerned about anyone who drinks more than that, the fear is that you can wash out too many nutrients i guess
(started at 266, down to 234, 34 to go...)