It all falls back on how well you trust your lightbulb (which is absolutely NOT a coast guard approved sparkproof/explosionproof device).
We had a Correct Craft that stayed in the water year round. We put 2 100watt infrared heat lamps under the engine block and put 2 quilts over the engine box every winter. Never had any problems whatsoever. Lake froze heavily one year (7" thick in the middle of our cove) but still no problems.
On the flipside, one winter we had heatlamps in the bilge of our houseboat. Power dropped out for 20 hours in 17 degree air temp with water temp around 40 degrees. Lost both engine blocks, 2 out of 4 exhaust manifolds, both oil coolers and the genset exhaust manifold...
I currently have the Formula in drystorage in a commercial marina shed - no winterization at all. Water still sitting in the block I reckon...
The SeaRay has a thermostat heater in the bilge set to 45 degrees, portable thermostat heater in the cabin set to 45 degrees, reverse cycle heat running on the rear unit at 55 degrees, "Fan only" running on the front unit. When water temp reaches 40 degrees, the rear unit will error out and quit. Front unit will keep running on fan only. That will keep air circulating. Little heater will do the rest. Water system full, but pressure lines drained to "lowest faucet" (the shower head lying on the floor of the shower) and all faucets left open for expansion. Pink antifreeze in the toilet and holding tank.
Been doing it like that for 3 years (before that I drained, filled with antifreeze, and everything like it was going to Antarctica for 6 months).
Location Tennessee - It's a low of 19 tonite with a windchill of 6.
I also keep a digital thermometer on board with hi/low memory function for each major compartment. After a COLD night I'll go check em and see how much leeway I have.