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.)
It should but I would drain it first and then how ever much antifreeze reaches the core will be undiluted. If little or none gets there, then no fear of freezing anyway.
don't count on it, I ran 5 gallons through my engine. spring came and i had a nice suprise.4 breaks in my core. disconnect and blow air through it after your done.
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.