When we built my trailer in 1997 we decked it with 3/4" Pour Form Plywood. This is plywood used to make concrete foundation forms. Its resin coated on one side, and the smooth side is soaked in some sort of oil product. When I rebuilt the trailer 10 years later the deck was 97% intact and strong. The bolts holding it down seized so I tore a lot of it up removing the deck to replace the steel ribs below. I re-decked it in the same product and it still looks brand new today, 8 years later using no preservative products.
For marine plywood I'd use end cut preservative available at most building supply centres and treat the entire piece. Some are green, a lot of the new ones are clear.