Normal resin is water proof as well to a certain extent. Its not like if you make a mug out of Poly resin that it will leak when filled with water....afterall your boats hull was made with polyester resin. The difference is that polyester resin isin't AS waterproof as epoxy and that overtime water can penetrate the poly resin easier then it can epoxy resin. That does not mean a solid fiberglass laminate will get soaked with water, it also does not mean properly sealed and maintained piece of glassed over piece of wood will get rotten.
Problems occur when screw holes aren't maintained and rebedded with sealant every couple years...stress cracks develop and aren't repaired, new hole and fittings are installed into the deck without proper sealant, etc ,etc. Water gets in there and although it may be a specific location in which it penetrates, it works its way all over through capillary action....rotting out wood cores in decks and transoms all over the world !
Epoxy however has the distinct advantage of being slower cure and therefore allowing greater saturation of the wood. Plus the epoxy itself is a different chemical composition which makes the wood FAR more resistant to rotting then simply poly resin. As a matter of fact, wood coated with epoxy probably will never rot.
Epoxy is expensive, costing about twice as much as poly resin, but for some applications its work it.