Bring your own water

If you cannot, then make sure the steward opens the water bottle in front of you (they often refill the "Evian" bottle in the back with tap water)

U.S. Says Drinking Water on Some Planes Contaminated
WASHINGTON (Reuters) - A surprising number of drinking water systems on domestic and foreign commercial aircraft tested this summer by the government did not meet federal standards because they were contaminated with potentially harmful bacteria, regulators said on Monday. [Reuters: US Domestic News]