I advise my local guys (NC) where boats are mostly left in the water over the winter to start their engine up maybe once a month to get oil smeared around in cylinder bores and to knock any corrosion off valves. Just start them up and raise idle to say 800 for a couple minutes. No need to warm all the way up as dockside that simply will not happen.
I have seen rusted cylinder bores and rusty valves. I don't worry about the bearings, or engine sludging up or fuel dilution in a few five minute runs.
Very important on Detroit two strokes as a few cylinders will really breath when sitting. A little block heat on the DD's solves the liner corrosion issue.
If boat is fully mothballed for the winter, then don't worry about it. Usually up north the air is super dry in the winter and the corrosion issue is much less.
Down here we get wild swings in temp and humidity during the winter. When a warm front comes, right before is a good time to get a little heat in the engine. Otherwise when the warm air comes in, engine looks like a cold beer in August. Dripping wet.
Again, block heat helps but the arctic 1000w units are super overkill. No need to keep it 120F. 250w will keep it 10-20F above ambient and condensation will not happen.