The problem with those space heaters is that when they run over 80% of the breaker capacity, they don't trip the breaker but the breaker guts, and the wiring get hot enough to melt. It's what we call "boiling the frog". Also, the key difference, is that at home, you're close enough to shut the heater off, smother the fire, call the fire department, etc. Your boat is 30 minutes away, and the staff may be gone for the evening.
The analogy being, that if you place a frog in a pot of warm water, and slowly turn up the heat over time, the frog will boil alive and not even realize it until it's too late.
A space heater with a thermostat is a safer alternative because you can set it on high, and dial it down so that it cycles on and off, preventing the cable and the breaker from "boiling".
When I was experimenting with charging equipment for my electric car, I melted down a few charging cables by running them over the 80% threshold.
On my boat instead of running a space heater, I just place 75 watt worklights in a couple of strategic spots. This keeps the bilge warm and dry, and the thru-hulls warm so that water doesn't freeze inside of them. It doesn't make the cabin habitable for human beings, but it does protect the equipment.
I apologize if I came off as being a smart-assed know-it-all, that wasn't my intent. I've really enjoyed your posts, and I'd hate to see anything happen to your boat.