The 737 coffee maker is plumbed directly to the potable water system. Ditto for the A320 and I'm presuming all the other Boeings in the fleet.

I'm not familiar with all the Express carriers and fleets but I have witnessed the FAs on ExpressJet ERJ-145s filling the small tank in the galley with bottled water.

I do recall when UAL first starting using Starbucks that Starbucks mandated a specific cleaning schedule for the makers/pots/tanks (this was mentioned in the internal announcement about Starbucks.) The coffee was fine until the spam hit the fan post-9/11 and anything that cost $$$ was dropped. Frankly, I'm surprised Starbucks put up with UAL as long as they did.