i recently replaced a 4X8 on my small utility trailer, that was starting to show signs of rotting in a few areas, this trailer sits outside all year in snow and rain as I live in the midwest. the original board which i suspect was a marine board(but i'm not 100%) lasted about 20 years.
i went ahead with a marine board replacement which i purchased at Menard's. Pricey yes, but i don't want to have to do it again in half a dozen years or so.
also, used stainless steel bolts and treated the entire board both sides with some decking water proofing i had left over.