RE: resins, sealants, fillers, etc
Yes, Silicone sealants can be used to seal GRP hulls (also known as fiberglass) and to attach equipment in them.
he thing they're not good for is for places where you'd want to paint or to attach wooden structures like decks. Paint or normal glue won't stick well (if at all). Silicone goop is good at sticking to things that are not easy to stick to, but very little will stick to the silicone stuff once you've used it.