You have several choices depending on how much you want to spend and how much work you want to put into it. Best choice is 3/4" plywood with fiberglass resin coating. Second choice could be marine plywood but I would still coat it with resin. You could use Thompsons but the stuff takes a long time to dry and if you use too much the glue for your carpet may not stick. Treated wood will work if you isolate it from the aluminum. Use some kind of calking silicone etc. Even if yoiu don't, it will take several years to react (if at all since only the edges will have aluminum contact. I know some may disagree. When (if) you carpet, use a good outdoor/waterproof glue. Sticks better and helps preserve the wood. Make sure you reinforce under the deck if you put a seat up there. Best of luck and enjoy the project..

Like2Fish,Thanks for the advise

ive used reg 3/4 plywood just use a good primmer and paint it after you screw it into place and use a construction glue a pl200 or better than put the carpet down at least 2coats of each this will last around 10years if you keep it coverd or under a roof and its cheaper than you think get a good oil base paint a mistint from anywhere color should not matter and add support for the chair a hard wood and prime paint and predrill holes and use glue works great been there done that and still using it good luck

ORIGINAL: eyesandgillz

Trout,
Quick question, was the pitting visible from the exterior or did you only notice it along the edges of the transom?

There are holes visible from the outside.  It ate the whole way through.  It's kinda visible on one of my initial pics on the first page.

All my best ideas came by PM and text message.

Good idea Trout About...must be nice to "be in the woods for work".