It seems even the manufacturers can't agree on this one, extract from the Webasto install manual and the way Webasto trains technicians, as does Mikuni.
"If there are battery isolation switches in the boat: connect the plus lead from the heating system directly to the positive pole of the service battery. This makes it possible for the heating system to always be switched off afterwards"
I prefer to err on the side of caution and have no means of accidental switching off in full burn, especially with water based systems, you might know about the dangers of switching off at the battery isolator but guests, prying kiddies and subsequent owners may not.
