Drinking too much water can lead to hyponatremia, water intoxication, water poisoning and death

Liquid H₂O is the sine qua non of life.  
Making up about 66 percent of the human body, water runs through the blood, inhabits the cells, and lurks in the spaces between. At every moment water escapes the body through sweat, urination, defecation or exhaled breath, among other routes. Replacing these lost stores is essential but rehydration can be overdone. There is such a thing as a fatal water overdose.  

The poison is the dose

Water can cause the brain to swell 

share courtesy: http://news.bbc.co.uk

Drinking several litres over a relatively short period of time could be enough to cause water intoxication. Those most at risk include people taking ecstasy, as the drug increases thirst and facilitates the release of anti-diuretic hormones so more water is taken in but cannot be excreted. Also, elderly people because their kidney function may be impaired. 

Strange but true; drinking too much water can kill.

In a hydration-obsessed culture, people can and do drink themselves to death. 

As long as you are healthy and equipped with a thirst barometer unimpaired by old age or mind-altering drugs, follow Verbalis's advice, "drink to your thirst. It's the best indicator." 

Beware of mindlessly drinking several glasses of water per day without considering your diet, exercise habits, climate, and sense of thirst. And when you do find yourself in need of water, remember that you can get it from liquids and/or whole foods that are rich in water.

Forcing your body to accept a large amount of water within a short period of time - say, an hour or two - can be fatally dangerous to your health. Here's why:

If you force large amounts of water into your system over a short period of time, your kidneys will struggle to eliminate enough water from your system to keep the overall amount at a safe level.

As your circulatory system becomes diluted with excess water, the concentration of electrolytes in your blood will drop relative to the concentration of electrolytes in your cells. In an effort to maintain an equal balance of electrolytes between your blood and your cells, water will seep into your cells from your blood, causing your cells to swell.

If this swelling occurs in your brain, you'll experience increased intracranial pressure i.e. your brain will get squeezed because the flat bones that make up your skull don't provide much give. Depending on how much water your drink in a short period of time, you could experience a wide variety of symptoms, ranging from a mild headache to impaired breathing. As occurred in the tragic water-drinking contest, it's quite possible to die if you drink enough water in a short period of time. 

Water intoxication, also known as water poisoning or dilutional hyponatremia, is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside safe limits by over-hydration.

Water, just like any other substance, can be considered a poison when over-consumed in a specific period of time. Water intoxication mostly occurs when water is being consumed in a high quantity without giving the body the proper nutrients it needs to be healthy.

One of the many Risk factors, is  Psychogenic polydipsia is the psychiatric condition in which patients feel compelled to drink large quantities of water, thus putting them at risk of water intoxication. This condition can be especially dangerous if the patient also exhibits other psychiatric indications (as is often the case), as the care-takers might misinterpret the hyponatremic symptoms.

Following sites referred thankfully and reference for further detail:

wikipedia.org

http://www.scientificamerican.com

http://news.bbc.co.uk

http://drbenkim.com/drink-too-much-water-dangerous.html