I think you may not have as big a problem with the boat in the water as you think. My boat spent 24 seasons frozen in on a mooring before I bought it. There was damage to the trailing edge of the rudder at the water line. Only because one winter huge ice bergs floated by and hit the rudder when the river broke up. Sailboat hulls are rounded so the pop up out of the ice as it tries to squeeze. My boat has a saildrive too and that had no problem. I'd close the seacocks and fill the hoses with antifreeze, then briefly crack the valve so the valve body fills with antifreeze. Don't forget antifreeze in the engine and wet lift muffler.