You know, 400 years ago, when tobacco was first brought to Europe, there was a debate that went on for decades about whether snorting a pinch of snuff was breaking a fast. Things change so little over the centuries!
I'm not sure what a water fast is supposed to do for you. But I suspect it has to do with giving your digestive tract a rest. (I'm not sure that's the best idea, but that's a different question, isn't it?) In any case, marijuana has nothing to do with your digestive system. It's not food (unless you're eating it. As some do! I do!) The only problem I can think of is that it makes you hungry so you're less likely to finish your fast.
Marijuana has no effect at all on colds, flu, fever, chills, broken bones, lacerations of the skin, etc. etc. But it does help you feel better until these conditions go away. So moderate use of pot might help you in a water fast.