To Drink or not??????????????
Potable water starts its life as the normal tap water in most countries, but, it is a question of how long it sits in a water truck before being dispensed to an aircraft. Then how often are the water tanks drained and refilled, daily??? I doubt it very much.
Like Blacksheep, I have seen inside, and had to clean out a/c water tanks during 'C' & 'D'checks. Even with regular hyperchlorination there is still a layer of gunge to be removed.
Also, remember, the water is not boiled in the coffee makers, (hence the often rubbish tea), so you will not be 100% safe from that source. If you want to go down that route, hope for a 'hot cup' on board. Better still, take your own bottle of water, where still allowed.