has to be water or it can be green tea as well. after ur first 2 weeks u will be allowed to use a CD water flavouring which u can buy from ur CDC. it comes in orange flavour and berries (i think). u can only use one spoon a day but that makes upto 1 litre and it tastes nice.
coffee/tea and juice wont count as they will retain ur water and u need to be flushing it thru. u will soon get used to the taste.
if its that bad tho...why not invest in a brita filter? thats what i use and it makes the water taste really nice