Question: what is the difference between pressure treated lumber and marine lumber?
Answer: The pressure treating process will weaken the strength of wood by as much as 15%, so if strength is the goal use marine. If rot is an issue, use pressure treated. Most plywood is 5 ply but marine plywood is 7 or 9 ply with no voids and using waterproof glue. marine is stiffer and less prone to warping.