Just picked up a boat with a heater and never had a heater before. The water circulates through heater core at all times, correct? So if running antifreeze through engine, heater will be winterized too? ('08 raw water cooled Indmar MCX.)
I agree with TigeMike and Steve. Drain that core first so you do not have to worry. The engine can suck up 5 gallons fast and the hoses to the heater core are very long and may not see all of the antifreeze.