An A320 Pilot reports about a deferral of the potable water system that did not comply with the MEL procedures for their A320. Captain also asks 'if the aircraft had a frozen water system as originally expected in the first write-up; how can one drain a water system that is frozen'?

Narrative

Aircraft was presented to Crew in ZZZ with a deferred item open; stating the forward coffee maker and spigot were inop'ed at a previous Station and the deferred MEL included draining the A320 of potable water and deferring the entire water system. While reviewing the Maintenance Release; the First Officer reported the forward lavatory toilet flushed normally. Water (H20) quantity was checked and it was found to be full; in direct contrast to the MEL instructions for the deferral on the Maintenance Release. Maintenance Control was contacted and a Maintenance Report item sent. Contract Maintenance called to aircraft in ZZZ and he reinstated the water system as no faults could be located. A few other write-ups were handled by ZZZ Maintenance Technician and he left the aircraft; with the Deferral placard still located on the forward Cabin Intercommunication Data System (CIDS) panel. This item was not discovered until en-route to ZZZ1. Aircraft was not serviced with potable water in ZZZ; so it operated at least one leg in violation of the MEL. The ZZZ Maintenance Technician stated the aircraft appeared to be not configured correctly for the 'No' potable water operation as all the valves had been left open. If the aircraft had a frozen water system as originally expected in the first write-up; how can one drain a water system that is frozen? Rhetorical question but that was the procedure listed under the MEL.