The previous owners son got back to town and confirmed the boat has never had any kind of heat in the engine compartment since they bought the boat in the early 1990's.
Long term power outages here aren't a problem like they might be in a heavily populated area. At most the power goes out for a couple hours at a time. This may be due to low population numbers, a large aluminum smelter in town, and what electricity the smelter and town doesn't use gets sold onto the grid, so there are some serious reasons to keep things up and running.
The PO's son is a mechanic and his Dad was a structural engineer, so I think they knew/know what they're doing...I'll be chucking something down in the engine compartment before it gets to -20C, just for my own peace of mind.
The brackish surface water does freeze here in quiet areas, but the underlying salt water never has. The channel does steam spectacularly from the temperature difference with the air at -20C though! Big time beautiful at sunrise and sunset