Fiberglass is technically just glass fibers which is the reinforcing part. When coated with a resin it is now a composite and becomes the hard plastic stuff used in boats and Corvettes and other things. The resin choices are polyester, vinylester and epoxy
. Poly is the cheap
stuff and very easy to work with. Over 90% of all boats use this. Epoxy is the good stuff and much stronger and only high end manufacturers use this. Vinyl is in between. Poly is not waterproof and requires an epoxy bottom coat. Vinyl is being used in many boats these days as an additional waterproofing barrier on top of the poly (which would be the outside bottom) for the hulls. You still need an epoxy bottom coat though. Many home built boats use epoxy and when properly done results in a really well made boat. All of the resins are not UV resistant and will degrade in the sun so you have to paint
your boat and urethane is a popular choice for this.
A fiberglass boat may have other non-glass fibers like Kevlar or carbon but it's still called fiberglass.