Yay, yes, water good. That's one of the most likely issues to be struck by people increasing the amount of exercise they do... Water balance gets thrown out, for a variety of reasons. Breathing causes water-loss (dry air into lungs, water laden air comes out), as does sweating. Also note - salt balance is important. Your kidneys control whether they let out salts or keep them in based on bloodstream concentrations, but sweat always has salts in it. This is the "electrolytes" that they talk about in "sports drinks". A bit of salt (whether in food or drink or whatever) is good after sweating, because you need to make up the salt from somewhere.