While we constantly hear about how healthy water is for our systems, too much of a good thing is possible. Drinking lots of water in a short period of time, say a few hours, can lead to "water intoxication." Although rare, this can be quite dangerous, and even fatal, if not treated promptly and properly.
While our kidneys are capable of processing about 60 glasses of water a day - almost four gallons - most people need nowhere near that much. Instead stick to simply drinking small servings of water throughout the day, and be conscious of drinking more if you are in a very hot, dry climate or participate in endurance sports.