Kept many an engine from freezing with the ole lightbulb trick. Use to keep one clipped to the underside of the engineroom hatch all the time. Everyone did. Use to keep it on year around. That little bit of heat will keep things dry and corrosion nonexistant. Old trick from way back.
Not to worry coors, a cover with light bulb will do just fine as long as there is no fire hazard. But, if the low nite and day temps persist for more than a day and a half, ya better drain it.
I've seen the marina put boats back in the water to keep them from freezing, when a cold snap in Nov. occurs before they have finished winterizing them. Also, I did the lightbulb trick for 4 days last fall when it went into the low 20's... no problems.
