A friend had a kidney problem and was told by her doctor to drink 45 litres of water a day. The poor girl was drinking water every available minute of the day, and peeing just as much. After a couple of days she went back to the doctors and told him she couldn't drink the 45 litres a day. He just laughed " I said 4 to 5 litres a day".
And honestly, this is a true story.
8 glasses yes, but I have recently discovered that my food and drug administration says 8 glasses of ANYTHING (as long as its not caffeinated).
I think though that sugar and carbonated liquids would not be that good either.