The water heater in my 2000 380DA lays horizontal under my mid stateroom sofa / bed.
As such, all the fittings are on the end, leaving a layer of water on the bottom when the heater is drained.
When I needed to setup the boat for winter storage, that ~gallon of water caused problems. If / when I ran pink / potable water antifreeze through the water heater, then that diluted the mixture, providing less protection for the hot water pipes and fittings.
The winterization that I found to work best was to swap the plumbing so that pink antifreeze poured into the empty potable water tank would NOT go into the water heater because my modified plumbing bypassed the water heater and instead go directly through the hot water plumbing and fittings.
This modification gave proper 100% protection, not only for the cold water circuit, but also for the hot water circuit.
To protect the water heater I have tried several methods and didn't like any method.
One method was to run pink into the "drained" water heater, resulting in a diluted mixture w/ less protection.
Another method was to remove the salon sofa, the water heater and stand it on end, to fully drain the tank. That was as huge a butt pain as it sounds, but the water heater was bone dry, w/ no freeze damage possible.
My water heater still has the original element.
FWIW, I made a check list to ensure I didn't forget any winter setup steps, because there were so many things that needed to be properly completed.
Now that I boat in the tropics, all that fun is history for me...