The Guardian had something on this today
"What people don't need to do is take in two extra litres a day," says Goldfarb. "You're going to take in two litres a day based on your diet and thirst sensation. What [bottled water companies] are really asking people to do is take in four or five litres, because they're already taking in two or three as coffee, tea, soft drinks, fruit, alcoholic beverages – that's all water. This notion is a marketing ploy."