Basil is totally right. Please do not drink unboiled water from an aircraft tank as you might be risking serious gastrointestinal infections!
As correctly stated, you never know where this water might have uploaded. Moreover, the effect of chlorine soon disappears so basically you end up with a mass of untreated water, fertile breeding ground for the bugs.
To say it all, IATA lays down rules about the quality of the uplifted water, tank cleaning schedules, periodic testing etc. but many airlines (and I know this from experience) ignore these regulations or are not even aware they exist!