FWIW, I tend to actively avoid drinking anything (so coffee or tea) that is made with water from the on-board supply. There is, I would suggest, a reason why in most galleys the crew all each have a big bottle of mineral water with their name stuck on / written on it.
Obviously water is also needed for toilet flushes & the sinks in the lavs. Am 99.99% certain the toilets have their own water system for obvious reasons. The water in the sinks is drinking water, so is probably connected to the same supply as the galleys.
I know at work when we had a water supply failure we all got sent home after about 2hrs as the buildings water tank on the roof was empty and there was no facility to wash your hands after a visit to the toilet, which was deemed a H&S risk.