It is 3/8 plywood.
The entire deck is 3/8 plywood and with the exception of the stanchion the soft area is not under a high stress area such as a windless. My plan is to use fiberglass where the stanchion mounts. Your question does bring up something I am wondering about. The soft area is in the same neighborhood of where the bowsprit mounts. I will know more once I cut but it appears the area where the bowsprit mounts is solid. At least I am hoping because there is a glassed in raised pad (4" x 6") where the bowsprit mounts and this will complicate things. If the area below the raised bowsprit pad turns out to be soft would you use plywood in this high stress area or fiberglass? There is a nice heafty aluminum backing plate. If the raised glassed in pad for the bowsprit mount also turns out to be soft (hard to say what was glassed under there...solid block of wood?) and has to be removed I have considered not trying to reglass in a new raised pad but having an exterior pad for the bowsprit to mount to. That said I am open to ideas.