I'll be doing a new engine hatch myself this winter. I WILL spend the money for marine plywood. A) Marine plywood uses waterproof glue unlike outdoor plywood which uses water resistant glue. B) Marine plywood generally has more ply's than pressure treated making it dimensionally more stable. C) Marine plywood is laminated from "premium" veneers so that there are no voids and, once again, premium venners are dimensionally more stable. D) As was mentioned, encapsulation is a good idea, but WILL NOT eliminate moisture from entering the wood and when moisture enters, it may never dry out promoting dry rot. Non encapsulated marine plywood will, based upon my experience, last 5 to 7 years with reasonable care.
So, do yourself a favor, call your nearest specialty (they sell hardwoods typically) lumber yard and get Marine Plywood.
BTW - No barn yard or domesticated animals were harmed in the creation of this response, however, if this response is directly responsible for the cutting of very pretty fir trees, mahogany, teak, or other rain forest inhabitants that result in the creation of wooden boats, go hug a pine tree in someone elses backyard.