This has nothing to do with the catering trucks. The potable water for the coffee makers and skin contact, e.g. sink is delivered separately from those carts.

FT has this notion that every complaint has to be about wringing compensation out of a carrier.

This one is about making certain that UA knows that a service failure ocurred. I would file a short (2 sentences max) complaint which simply advises that potable water was not delivered and the aircraft departed without it. I would expressly state, "I don't want anything from UA for this service failure, but I would appreciate it if you would follow up to determine how the aircraft departed without proper service."

While the failure to top up the tank was not the FA's, it was the FA's responsibility to check that the tank was at an acceptable level prior to departure and to notify the Captain and then ground services when it was not.

The failure to follow pre-flight checklists here had no ramifications other than minor nuisance. But, failing to stick to a pre-flight checklist means that the FA's likely overlooked other items and some of those relate to safety.