It looks to me that your average water temperatures likely do not get below 40F in the winter: US NODC Coastal Water Temperature Guide
Your boat is at a slip, presumably with shore power. You will want to check on it from time to time after any major weather systems come through. Check the light bulb when you go to check on your dock lines.
The real worry with a gasoline engine is if there is a gas leak, even a slow one. With some heaters that have exposed elements this could be a recipe for disaster. I'd recommend shutting off your gas line even if you decide to use a 75W bulb. You want to keep the boom on the mast and not in the cabin!!!
From what I think I know about your climate in Oriental, NC I don't think you have much to worry about. You could try asking some power boat owners (usually gas engines) what they do to 'winterize' their engines. I'm guessing that 20F cold snaps are pretty rare and extremely short lived in your neck of the woods. Even during a cold snap the water your boat is sitting in will be much warmer then the air which will keep your boat and engine warm enough.
I've seen LI Sound (salt water) produce ice around the shoreline but that takes about 2 weeks of temps in the 20-30F range but that was 40 years ago. NC has the Gulf Stream off of it's coast which helps keep the sea water warmer while we have the cold water Labrador current bringing cold water down from Newfoundland.
Given all of this I'll suggest that even a 75W bulb in the engine compartment would be overkill for the winters you normally get in Oriental. Don't trust me though, ask some local boaters what they do.