As I'm sure you know, broadly speaking polyesters fall into two categories Isophthalic and Orthophthalic; boat building leans heavily onto Isophthalics especially in the Gelcoat and outerlayers of the hull, deck and any other exterior laminated parts because of its extremely high resistance to Osmosis (water absorption) that blighted early GRP boat production. There is definitely a move in the marine industry to move away from Orthophthalic entirely.
You do get marine specific epoxies however with varying degrees of fillers and other materials or 'hybrid' resins such as Durepox which is an 'Epoxy Urethane' to take advantage of strength characteristics.
The other reason I failed to mention in my first response is that most hull construction still heavily relies on Chopped Strand Mat, normally emulsion bound and thus epoxy isn't suitable because it won;t break down the emulsion binder.