Drinking too much water can lead to hyponatremia, or water intoxication, a condition in which the blood is diluted, causing serious illness or death. However, a person would have to drink a truly excessive amount of water to begin showing symptoms.
When a person drinks too much water in a short amount of time, the kidneys cannot flush it out fast enough, leading to an accumulation of water in the blood. The excess water enters cells around the body, including brain cells. When brain cells take on too much water, they begin to swell, which can lead to seizures or an embolism.