> Suppose you were to be building a stripped deck, but weren't putting epoxy
> down on it. It was just bare wood. And of course you didn't want it to get
> waterlogged. Is the varnish enough, or is there a better material? Heck,
> would polyurethane work?

Varnish is not recommended for below the waterline in boats that stay in the water all the time. This suggests it is not 100% waterproof. However, for topsides it is good protection. The varnish should keep your wood from getting wet.

Remember that the fiberglass contributes significantly to the strength and durability of a stripped panel. I would be wary of leaving off the fiberglass. Many boats were built with wood before fiberglass and resin came along, but they had ribs.