Polyester and wood are incompatible, use epoxy.
The interior of a barge can be difficult to access. If you leave it bare wood you can spray it from time to time with a borax wood preservative dissolved in fresh water.
"Some number of years ago there was an article in the wood technology section of WB addressing the use of disodium octaborate tetrahydrate
(DOT) as a safe wood preservative. DOT is a boron based water soluble salt that is lethal to wood rotting fungus while being only 1/10 as toxic as table salt and 1/8 as toxic as aspirin to humans and other life forms, excepting those that use said fungus to digest their food, namely termites, carpenter ants, wood boring beetles and cockroaches."
You can read more about it at www.copperbrite.com