Many builders don't care fore the cost of epoxy and I can't say that I blame them. Stuff is not cheap.
Many search for cheap alternatives. Polyester resin is often selected as one of them.
Epoxyworks (West Systems' newsletter) did a study about effectiveness of different coatings at keeping moisture out of a dried piece of plywood. The study had some interesting results.
I'm not sure how long the article will be there so I'll summarize. In order below, with 1 being the best and 7 being the worst, these coatings rank as follows:
1) West System 105/205
2) Vinyl Ester
3) Low Performance epoxy systems.
4) 2-part polyurethane paint
5) Polyester resin
6) High-solvent epoxy penetrating systems
Now this is a West System company study so I'm sure they are not telling what they consider a "low performance epoxy system" to be. But my gut tells me it is any epoxy system with high mix ratio of like 4:1 or 5:1.
What I found most interesting was that 2-part polyurethane paint holds out moisture better than polyester resin!
Now we must take this study with a grain of salt. For paddle craft that don't stay constantly in the water, the importance of moisture exclusion decreases a bit.
But bottom line: for the best protection, you can't beat epoxy.