I have done several long fasts (30, 45, 60 days) and many shorter ones, mainly for colon cleansing purposes. The 60 days I did The Master Cleanse to day 48 when I experienced True Hunger for the first time (!), and my wonderful naturopath switched me to fruit/veg juices to day 60. I have always had amazing energy on the Master-Cleanse and juices, continuing my regular gym routine every day barring detox crises, with energy to spare.
The big difference I am experiencing with water is the lack of energy. I mostly lounge around, even napping! I feel fine, very mellow. I am drinking water with the inspiration of many things I have read, notably falconblanco and the International Hygienists. I feel that I am clean enough at this point to Water Fast without hurting (!) and am interested in the Deeper Clean described by so many.
This is the end of day 4 for me, and I get great inspiration from those of you drinking water and continuing to function past day 10! Go get 'em!