It is possible to drink too much water. While normally functioning kidneys in an adult can process anywhere from 20 -1000 ml of fluid per hour, it's not easy to overload them -- but it can be done. Excessive amounts of water, especially over a short period of time, can be life-threatening. Symptoms of too much water include weight gain, bloating, nausea, and vomiting. Acute cases of water intoxication can cause low blood sodium, which can result in headaches, confusion, seizures, and coma.
The International Marathon Medical Directors Association recommends that athletes drink no more than 31 ounces of water per hour during extended exercise.