Hi - my advice is that you empty the motor from all seawater first when the boat is on hard. Then you put in bucket 50/50 tap water/Zero100 glycol and insert inlet to bucket. Then you insert cooling water outlet also to bucket in order to reach full circulation between bucket and motor. You may keep circulation about 10 - 15 minutes and then motors' water space is full of antifreeze. Then you loose outlet from bucket and insert it to rubber outlet to ensure that the muffle will not freeze.
Good wintertime