Turn off the water pump
I used plugs to jam the hoses without draining the whole water tank.
I did not find any valves to disconnect the heater
You can change the heater inside the bunk compartment leaving the
hoses from the engine
connected, but I would suggest to
go the extra mile, get it out completely and rinse it with fresh water.
Inside is a lot of rust and remains of the old anode, I would definitely
go the extra mile and clean it before re-assembling.
You can let the heater drain to the bilge
, the bilge pump
will take care of the water automatically. Also there is a over-pressure vent that drains into the bilge