Antifreeze
You don't say which model engine, but you should be aware that there are two types of cooling systems. 1. Older boats may be direct seawater cooled. 2. Newer boats are have a closed permanent antifreeze loop with it's own internal metal water pump, then they also have a strainer and sea water intake hose, typically to a belt driven impeller pump, heat exchanger and waterlift muffler for the seawater side.In both cases, after warming up the engine,and shutting it down, you need to disconnect the seawater intake hose from the closed seacock intake and put it into a bucket of at least one gallon of non-toxic -100 degree antifreeze. Start the engine and let it suck up enough antifreeze until you can see it comming out in the exhaust stream, then shut down the engine replace the intake hose on its seacock and change the oil and filters.You have winterized the intake hose, strainer, the heat exchanger and the waterlift mufler in the above process.However, you didn't touch the existing closed loop permanent antifreeze, so you might want to 'test' it at its filler cap or overflow canister to see what level of freeze protection is registering on the tester. Changing out the permanent side antifreeze is a little more involved and specific to each engine because of the location of the thermostat and drain cocks in the systems. I'd recommend an engine manual and using Yanmar's recommended permanent antifreeze. Do this first if you elect to take it on; then do the seawater or 'easy' side. Sorry I didn't put that in the correct order, but I don't change out the permanent antifreeze every year myself.