Well, plywood soaked in polyester resin with no CSM is not going to do as much good as if you had added the CSM. When the wood flexes, and it will, the hardened resin, with out the fibers of the CSM to hold it together, will crack open and allow water to penetrate to the wood fibers. Even though it's the bottom of the plywood condensation can and will occur and moisture can and will accumulate under the deck. IF you keep the boat covered and maintain her well, you should still get a good long life from your deck. The resin coating alone will still be better than nothing at all.