You might find this a little hard to swallow -- but it's good to have this approached in moderation for once. I found this on snopes.com:

Claim: 75% of Americans are "chronically dehydrated" because they fail to drink eight glasses of water per day.
Status: False.

The URL for this page is http://www.snopes.com/toxins/water.htm
Urban Legends Reference Pages © 1995-2004 by Barbara and David P. Mikkelson

I wanted to reproduce the whole article here, but it's copyrighted, so I daren't. Please visit it, at my request. ;o)