Excessive water in your system can dilute your body fluids so much that the sodium levels become life-threateningly low. Water sure can send your health into a tailspin, even endanger your life, if you drink too much of it.
Excess water dilutes body fluids so much that the sodium levels become life-threateningly low, causing cells to swell. That includes brain cells leading to loss of consciousness, seizures and even coma and death. For long-distance runners, this can be a particular hazard. With half-marathons and marathons being all the rage, runners while practicing must resist the temptation to tank themselves up with too much water.
Drinking more water than you need increases total blood volume and also pressures kidneys into working overtime so as to filter excess water out of your circulatory system. It is, however, incredibly rare for someone to die of dehydration in a temperate climate, not even sportsmen who sweat a great deal.