Source: The Comprehensive Guide to Wilderness First Aid, by Clifton Castleman, WEMT.

Water is an amazing thing - it keeps you going. Literally.

However, your body is made of up SALT WATER, not fresh water. By drinking TOO much water, you actually change out the composition of your blood from 70% saline to 50%-70% fresh water (called hyponatremia).

Not only is this dangerous, but it can kill you. There is, luckily, an exceptionally simple answer to this problem. Eat SALTY foods while you drink! You can drink as much as you want, so long as you constantly eat salty things and keep your salt levels up. You can also do this by drinking Vitalyte or other similar electrolyte enhanced solutions (elixir by Camelbak, Nuun, Gu, etc.). 

Gatoradeis NOT acceptable; not even when diluted with half water. Gatorade has too much sugar which will not only slow down the amount of liquids being absorbed into the bloodstream, but also could easily make you nauseous or even vomit! That's the last thing we want!

- How the Body Uses Water -