Pretty much every substance can be unhealthy or even lethal when taken in large enough quantities. This also holds true for plain water.
If one digests too much water, the level of sodium in the blood gets out of balance. There is enough sodium, but too much water. The consequence is a hyponatremia, or water intoxination. The possible symptoms are quite scary:
”Initial symptoms typically include light-headedness, sometimes accompanied by nausea, vomiting, headache and/or malaise. Sodium levels below 100 mmol/l (2.3 g/l) frequently result in cerebral edema, seizures, coma, and death within a few hours of drinking the excess water.” (Wikipedia)
In extreme cases it is even possible to die of water. This should be quite impossible through any sane consumption (so don’t panic), but every now and then some people indeed manage to die of water – usually in the context of strange contests.
For example, there was a contest where one could win a Nintendo Wii gaming console. Winner was the person who could drink the most without going to the toilet. A 28 year old woman died from this quest for a Wii. (see Gamasutra)
In 2005, a californian student died of water after drinking 20l mug of water. (see Abendblatt.de[German]).
In England, a 44 year old man died of water intoxination. He drank 10 litres cold water per day, because his gums were hurting. Using this “treatment” three days in a row, he finally collapsed. His brain was swallen because of the extreme water consumption and he couldn’t express himself clearly anymore. One day later he suffered a heart attack in the hospital an died. A court finally decided that the official cause of death was a heart attack because of excessive water consumption. (see Spiegel Online [German])
People participating in marathons seem to sometimes suffer from water poisoning when they take up too much water.
A already mentioned: These are extreme cases. Unless you engage in stupid drinking contests, it should be almost impossible to ingest a lethal dose of water by accident. So don’t thirst yourselves to death. 🙂
Unfortunately, I do not know a reliable source which tells which amounts of water can become dangerous. Some forums talk about 5-6 litres within half an hour (!), unfortunately without sources. Whoever has a good source: Please leave a comment below!