Water expands when it freezes solid. It will even crack or stretch solid steel in the process, fiberglass has zero resistance to the effect. Just like a Minnesota highway, each time it freezes it makes cracks. Each time it melts the water seeps in deeper, freezes, opens up the cracks, makes more cracks, and the cracks work deeper into the structure....freeze thaw, freeze thaw, pretty soon its busted to smithereens.
Tread carefully, whatever you do.