i agree with chris and would add this: it is very difficult to accomplish a true Water Fast with a raging addiction. the addiction will make you break the fast- just as you are contemplating doing right now. i am not talking about light users, here, i'm talking about serious addiction which only you can decide you have. deal with the caffeine addiction first, then do the water fast. it takes about three days to a week to get over the active physical withdrawal. i used the weaning method. switch to green tea, as it will allow you to get enough caffeine to avoid the serious headache, but allow you to decrease your amount of caffeine rapidly. after three days, go cold turkey the rest of the way. it works like a charm. then do your water fast.
if you think you can tough out the headache, fatigue, craving, etc. by going cold turkey immediately, then awesome! but it is hard, i know very well from personal experience. not impossible. just very hard!
aside from the toxicity that chris mentions, if you are drinking coffee, then you are not really doing a water only fast. you are doing a coffee only fast. when you think of it that way, is that really what you want to do to your body?