Your body needs water to function properly, but your body wasn't designed to get all of the water you need for a whole day in just one sitting. It's good to drink a large amount of water throughout the day, but drinking a gallon or two of water all at once is dangerous.
In fact, it can be life-threatening. Hyponatremia, which is sometimes called water intoxication, means "low sodium in the blood." It occurs when someone drinks enormous amounts of water and/or loses too much salt from the body in a short time.
Drinking too much water overwhelms the kidneys so they can't process and eliminate the water fast enough, and the levels of sodium in your blood can get too low. Severe sodium deficiency can lead to twitching, seizures and even death.
Hyponatremia can happen when marathon runners drink gallons of water and don't replace the electrolytes during a race, or when people with certain psychological disorders can't stop themselves from drinking water. It can also occur in older people and people with certain medical conditions.
A person with hyponatremia needs to see a doctor for diagnosis and treatment.
American Family Physician. "Management of Hyponatremia." Accessed February 9, 2012. http://www.aafp.org/afp/2004/0515/p2387.html.