I suppose that would depend on how you winterize your boat. If you cover over the carb or intake air port, sealing it, and your exhaust, you'll have gone a long way to keeping moisture levels down within the engine during the freeze/thaw cycles of the winter. Failing this, you can check the oil in the spring after a brief running of the engine, and if no white oily foaming is evident you probably have too little moisture in the crankcase to worry about. Changing it in the spring will hurt nothing, but is not necessarily required. If you keep a light bulb next to the block during lay-up it will provide enough heat to keep the engine warm and thereby prevent moisture from ingressing to the engine. You could use a dipstick block heater to do the same, if you are insanely wealthy. They draw about the same current as your microwave and, while I doubt you have much experience leaving that appliance run for three months continuous, I expect you get the picture. Probably about the cost of running a hot-tub through the winter. The 100 watt light bulb and a method of containing it's heat next to the engine will do quite nicely.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.