I think it's raw water
cooled only (no heat exchanger), so you must drain the system: unscrew the pump cover, remove the impeller, and drain the water lift muffler
. To be safe, I'd also remove the thermostat. Replace the cover without the impeller (don't forget to replace the impeller in the Spring).
If you're determined to just run antifreeze through the system instead, be sure the engine is completely warmed up with sea water running through it before you pump the antifreeze! Otherwise the thermostat will not be open, and you'll not get antifreeze to the block and head!
Good luck with the winterizing, and keep warm!