Yes, plastic. It''s the plastic resin that holds the fiberglass together. Thus the letters "FRP" you see at times, meaning fiberglass reinforced plastic. Thus also the term "Classic Plastic" you see in some sailing magazines.
The term "plastic" has come to cover a lot of ground of course. But it''s accurate to refer to fiberglass boats as "plastic" despite the negative connotation to some.
For anyone interested in a good read this winter, get Dan Spurr''s book "Heart of Glass", which is a history of the fiberglass boat. It''s good stuff.