When you run the 5 or 6 gallons of antifreeze through the system you will have winterized the mufflers if you see antifreeze coming out the exhaust. You are in salt water, that's hard to freeze. You do not need a liquid antifreeze to prevent damage. A slush will not crack anything. In my 10 years at Pt. Judith, R.I. I never drained my mufflers. Capt. George