Most locations do not sell marine grade Sande, however, in few and far between locations, it is available, and is a very legitimate marine ply.
Contact edensaw woods in NW Washinton State...they carry it.
Have no fear. I left a sht of 1/2 in sande marine outdoors in the pnw for 2 years. We were having fence issues in a corner I couldnt see so I threw it up and forgot about it. One corner was stuck in the mud. One surface weathered but in good shape. I sawed into the corner that was in the mud to see what happened. Nothing happened, glue was holding, it was in great shape. I also took some and boiled it with no problems.
I will say though I won't use it for stain / clearcoat surfaces. Only painted, below waterline is fine. The reason is it is not very pretty.
Also would not buy it from box stores. Only from a distributor that you know is selling marine grade. Out here we have architectural woods inc (AWI) and Edensaw. Sande ply comes in interior and marine. Box stores probably selling the interior type.