Yes it is a myth. It has been a very fashionable myth for some years now, but there are signs that some people are beginning to talk sense about the matter.

At the height of the hysteria, when people were shrieking at long distance runners that they were not drinking enough water, friends of mine who assisted at certain famous marathons told me that they had loaded runners into ambulances, unconscious, apparently because of dehydration, and yet the victims actually sloshed as they were picked up! These people really had drunk too much water, to the extent that they had endangered the lives and lost consciousness because of water intoxication.

At that time practically every general practitioner had young ladies come into the consulting room, burst into tears, and say that they did not dare go more than a few metres from the bathroom. They had to keep running to the lavatory to urinate every few minutes.  Almost invariably there was nothing wrong with their kidneys; they simply were knocking back water by the litre, sometimes even in the doctor's office! Someone had neglected to explain to these patients who had been instructed to beyond the bounds of their capacity for education, that what goes in must come out. That is a subtle and difficult point to understand, but it must be mastered in some form before one can claim to have a full, functional understanding of human physiology or, for that matter, basic physics.

It makes good sense to consume plenty of water in any convenient form, such as coffee, tea, guarana, cola drinks, soup, fruit juice, steak, vegetables, sports electrolytes, you name it. Even water! But a good time to drink water is when you feel thirsty. This is a new discovery of course, and if modern science had not revealed it to us, what on earth would we have done? Can you imagine the sufferings of Julius Caesar or William the Conqueror or Charles Darwin, who crossed their legs before the days when anyone had made that discovery?

It doesn't bear thinking about, does it?

The idea has arisen due to a misconception. We lose 6-8 glasses of water a day by perspiration, in our breath and in feces and urine. But that does not mean we need 6-8 glasses of water. Much of the water than we lose we gained from our food (in many cases 50% 0f the total weight of a meal is water) and more is generated by the body as a metabolic by-product. In most cases this will account for about half the total water lost, so you only need 3-4 glasses of water as a replacement. 

The health claims for large quantities of water are mainly unsubstantiated. The main result is excessive urination.

This all comes from a lack of understanding of a report by the US Food and Nutrition Board in 1945. The board suggested that people should consume one milliliter of water for every calorie of food consumed. Who knows where they got this nice round number from. Nutritionists then did a calculation which worked out at an average of 64 ounces of water a day. As has been pointed out by others, they ignored the fact that almost all of our food already contains water.

The Board had another look at this in 2004 and, after comming up with some average figures of total water consumption, concluded that people get enough water by letting thirst be their guide.

If the "tea and coffee doesn't count" thesis was true I would have died of thirst long ago and so would most Russians. Ind I guess most inhabitants of other countries where tap water should be drunk only after boiling and many people can't afford bottled water all the time.

Yeth, it ith a mith, the water only turnth to pith.