Factors such as the cleanliness and temperature of the water service vehicle, which brings water to each aircraft, affect microbial counts in tap water on airplanes.
A soft drink could be a good substitute for coffee or tea. However, you might want to think twice before accepting the cup of ice your flight attendant offers.
If you must have ice, the TSA will let you bring ice cubes onboard in a small plastic bag. Pour off meltwater before going through security and again before boarding the aircraft.