tl;dr – You weren’t drinking it anyway, right? Right?
I came across this graphic on LinkedIn depicting the results of an Airline Water Study conducted by the Center for Food as Medicine & Longevity. Here’s how all the major US airlines ranked when it came to water quality:

Linked in user Manmohan B. indicates that the “three-year study analyzed 35,674 water samples and found E. coli violations were the strongest factor driving poor scores. Researchers also flagged weak enforcement of federal aircraft drinking water rules.”
Though bottled water is often provided on airlines for drinking, the onboard potable water systems are used to make coffee and tea, so if you order those beverages, you’ll be consuming water from that source.
In case you’re interested, here’s how several US regional carriers fared:
GoJet Airlines: 3.85 (Grade B)
Piedmont Airlines: 3.05 (Grade C)
Sun Country Airlines: 3.00 (Grade C)
Endeavor Air: 2.95 (Grade C)
SkyWest Airlines: 2.40 (Grade D)
Envoy Air: 2.30 (Grade D)
PSA Airlines: 2.25 (Grade D)
Air Wisconsin Airlines: 2.15 (Grade D)
Republic Airways: 2.05 (Grade D)
CommuteAir: 1.60 (Grade D)
Mesa Airlines: 1.35 (Grade F)