You can overdo the water. Like Erika said, you need the electrolytes. There are some recipes for home made drinks that will help that, so you don't have to drink that sugar-ladden Gatorade. Here is one:
http://www.earthmamasworld.com/homem...-for-kids-you/

Just type in electrolyte drink recipes and you'll see a ton. Most of the time you can just add some lemon, sugar and pinch of salt to water and that's good enough, but there are a lot of variations you can choose from.