Are you sure the cracks are in the fiberglass of the hull? Looks to me like the cracks are in the white vinyl trim channel that’s part of the rub rail. The white channel is screwed or riveted onto the hull, then the black vinyl trim is inserted into the trim.
If it’s trim, it is not keeping water out or adding any appreciable structural strength. It’s purely cosmetic. If it were my personal boat, I’d leave it alone, because it’s a big job to replace it and expensive. It’s not worth the work to me.
It it’s the trim, the right way to fix it is to remove the black insert. Remove and replace everything the white channel, and then re install the black insert
I’m not familiar with the boat, so I could be mistaken about it being the channel for rubrail. . But it sure look like it from the photos.
