As water freezes, the ice formed expands and can damage fiberglass structures. Keeping a boat well covered during the winter is a major item of proper maintenance. The cover should have a high ridgepole to lessen snow loads and provisions must be made for ventilation. Hatches or sidelights should be open to lessen the growth of mildew in the fall and spring. Don't forget to cover the mast and rigging.
Water can seep into the wire rope fittings and damage can result when the water freezes.
It doesn't happen instantly but could take several years to become a problem. Grease the turnbuckle threads to preclude water entrapment. Leakage around poorly bedded deck fittings seeps into the plywood, balsa, or foam core. Winter comes along, ice forms and expands in the core, and the fiberglass separates from the core material. This is commonly called delamination.