When you drink too much water, you will get abnormally low blood sodium level. You can get very sick.
Do not over drink when you do intensive exercises. During such intensity, the kidneys cannot excrete excess water. As people keep drinking, the extra water moves into their cells, including brain cells. The engorged brain cells with no room to expand will press against the skull. This can compress the brain stem, which control vital functions such as breathing. Worse case scenario when this happens is dealth.
You can balance your water intake with sodium. Take 1/4 teaspoon of salt per quart of water (that is every 4-5 glasses of water. The best salt is NOT table salt but Celtic or Himalayan seat salt which are available at health food stores.