White Oak or Mahagony is the best option. Both are naturally rot resistant and will last 10 times longer than plywood. Plus they are stronger than plywood, except for marine grade plywood, or rather plywood with no voids.
The problem with plywood is the way its made. The cross layers add strength, however they alos provide opening (end grain), and have voids. The voids weaken the plywood, plus they will trap moisture and the rot will start inside the wood. You can put a protective layer on the outside off the plywood, however once that barrier is breached your wood starts to rot.
You can put a protective layer on the white oak, mahagony too, however once that protective barrier is breached there still wont be any rot. Both woods resist rotting. I just replaced a 40 year old transom, yes white oak. At the same time I replace a piece of plywood on the outside of the transom, and the plywood is less than 10 years old. The oak was still solid and strong as hell, the plywood practically fell apart in my hands. Only reason I replaced the transom is I was redoing the boat and the transom was looking mighty old and worn.