Regulations regarding cleaning of water cooking equipment on planes?
From observing catering operations as a passenger on many flights:


Hot water is served from the same pots as coffee, so that only one type of hot liquid container is needed per flight. The pots are clearly intended for hot water or coffee. This doesn't mean that the flight attendants haven't used it for something else. For example, the galley sink might get clogged and the flight attendant uses the pots as a scoop to move the liquid to the toilet. I've seen this at least once.
Yes, everything with food contact has to be washed and disinfected, especially on international flights for phytosanitary reasons. They even will wash the trolleys. They cannot disinfect the pots on the aircraft, so at the end of the flight, they are placed back in the metal catering containers, and will be washed in the catering facility with all other supplies ("rotables").
There's no regulation or non-religious reason why you'd want to separate the coffee pots from the rest of the equipment, so I doubt any airline does this. Here's a page showing pitchers being washed with serving trays and drawers.


In the majority of galleys, the hot water comes from the coffee maker or dedicated water boiler, which operates like a commercial coffee maker: there's a constantly heated internal hot water tank that is automatically refilled from the aircraft water tanks. My understanding is that it may be best to have hot water dispensed directly from the maker, since that is dedicated for water use.