If the temperatures will only be below freezing
overnight, back up to at least 40 F during the day, you have nothing to worry about. It's only when a hard freeze lasts for more than 48 consecutive hours that anything on board a boat kept in the water can get cold enough to freeze. In fact, as long as the water the boat is sitting in doesn't freeze, no matter how cold it gets, nothing below the waterline will freeze because the boat's hull takes it's temperature from the water and nothing inside the hull can get any colder than the hull. If you're expecting sustained temperatures--48 hours or longer--in the 20s F, you'd be well advised to winterize. Complete directions for winterizing plumbing are provided in the article "Winterizing Plumbing" in the Head Mistress forum Reference Library (right side of the forum homepage).