Any marine service dept would tell you that it can be fixed correctly. The fix is to level any breaks, clean out any cracks, fill with epoxy and sand. Apply matching gelcoat finish. If the crack is from a know stress situation (like from a corner or abutment) you could strengthen the crack location with fiberglass mat.
If you don't care about cosmetics, you could just fill the cracks with an epoxy. The stranded body filler works well for large cracks. Regular Bondo will fill the small ones. Poured epoxy is the lazy man route.