I've responded to so many water threads with this information, that I wrote a blog entry on it, thus the link about my mother's experience (and mine indirectly) with water intoxication, and advice from her kidney specialist on how much water people "really" need, and how they can get their water needs met (without drinking a single drop of anything, if they really wanted to). This information is on the web too.
Most of the water advice given out (especially to dieters) is total hogwash. The body needs about 2 quarts of water - in any form. That is that every drop of water counts - whether it's in the form of fruit, milk, soup, lettuce, juice, coffee, tea... Yep, even caffeinated beverages count because while there is a slight diuretic effect of caffeine it "cancels out" only a very tiny amount of the water in the beverages, and if you drink caffeinated beverages regularly, new research suggests that your body "outsmarts" the caffeine and there is virtually no water loss at all.
In tracing back the water myth of 8 glasses of water, the consensus from modern medical experts is that a professional paper decades ago was taken out of contests. The result of the research was that people need 6 - 8 glasses of water, not seperate from their food, but including every drop of moisture that is taken in from foods (even breads, grains, and cooked meats surprisingly are largely water) and beverages. Taken out of context, dieting "experts" quoted the 6 - 8 glasses, and misquoted the information as 6 - 8 glasses of fluids. Like a game of telephone, the message got further destorted to exclude caffeinated beverages, the water in soups, in milk and other nonclear beverages, and eventually everything but "plain" water. New (and even more dangerous) myths cropped up to advocate even more water intake, such as that larger people need more water than thinner people. This is dangerous, because obese people do not have larger kidneys and their kidneys are no more efficient than a thin person's. In fact, because of a greater risk of being on medications or having other health issues, obese and overweight individuals kidneys may actually work less efficiently than a thin person's, putting them at greater risk for water intoxication.
All of that being said, you might assume that I am against drinking much water. That isn't true at all. For weight loss, water can be very helpful. It may increase metabolism (research is mixed on this) and it can decrease appetite, at least temporarily, but if there is any reason that you may have low electrolyte levels or reduced kidney function (medication, low sodium diet, illness chronic or short term, diahrrea, vomiting, intense exercise...) then you really should consult your doctor before you consider consuming more than 3 quarts of fluid (caffeinated beverages and soup, etc included).