370 Motoryacht,the boat is a 1985 3607 and it has what appear to be factory installed heaters in the engine room, one over each engine and one over the generator with a thermostatic control for these located center aft of the engines in the engine room.
In addition there are also heaters installed in the aft cabin head and under the stairwell leading down to the galley, these are individually controlled by thermostats located on the heaters themselves.
I have the original paperwork, manuals, etc. that came with the boat as originally purchased so I have no doubt the boat came equipped with the heaters but don't know if they were factory or dealer installed as some of the documentation I have does refer to the dealer.
I do however know of a 1986 model equipped identical to mine which seems to support that it was a Carver option whether factory installed or dealer.