When you build a tank out of plywood and glass, you DON'T leave the plywood exposed to water... Booze is talking about what to do to waterproof the wood on the inside.
I've never actually seen this done, but it's either a marine epoxy that is used to give boat hulls that plastic-like coating, or it's fiberglass.
Personally, I can see the fiberglass working well for this... if I remember correctly, you paint the inside, then lay pieces of fiberglass cloth on the wet paint, then paint again over that, leaving you a seamless, incredibly strong, waterproof and completely saltwater safe/impervious surface.