Put down a layer of tar paper to help keep moisture from coming up off of the ground onto the bottom of the deck.
Used Advantech ¾" decking for the sub-floor. It's water resistant. Not water proof, but very resistant. Lots of examples on the web where people were able to leave it out in the weather for several years without significant swelling. We paid more for this than standard plywood but much less than marine plywood which was my other choice.