Many customers are under the impression that the aircraft potable water tanks are little more than mobile cesspits, with algae fronds waving around in the sludge and a few coke cans, dead birds and used condoms tangled at the bottom. Urban myths die hard and slow.
As a member of the inland pleasure boating scene, I think there is a certain level of paranoia being exhibited here. Most inland boats have water tanks made out of mild steel as part of the structure which are (usually) painted on the inside with a black paint (Water Tank Black). As a result many boats have the same water in their tanks for weeks (even months) this water is often consumed directly from the tank. As a boater for nearly forty years, I have never heard of a mass epidemic of water carried diseases in the boating community. Some newer boats have either stainless steel or Polypropylene tanks and I assume aircaft tanks are of similar construction, so I would not worry about the water having been in the aircraft tank for a matter of hours.
As has been mentioned above the very term Potable water means it is safe to drink. That said however, in some countries it is not always palatable. So if you are thirsty on board and offered water from the potable tank, have a drink, it might not taste nice, but it is unlikely to kill you.