I have heard references to gelcoat being a water proof application for a fiberglass boat hull. I have always considered fiberglass as inherently water proof. There are several industries surrounding fiberglass as a roofing product.
I know gelcoat serves several purposes, most notably it is smooth and UV resistant.
From a boat prospective; The gelcoat is usually applied to the mold, with the fiberglass applied next. To the best of my knowledge no one applies a layer of gelcoat to the inside of the hull. Which means that big fiberglass boat hull is an untreated bucket of water, it is protected on the outside from by the gelcoat, but nothing is protecting the inside. I would imagine in most cases, there is always some standing water someplace in most boats, most drains I have seen leave at least a cup of water in the boat, and I can't say how many I have seen collecting rain water.
What if any damage might soaking fiberglass in water do to the fiberglass?