[warthog5]

The standard is A-B Douglas Fir marine.

There is no diffrence in the wood in Doug Fir [DF] ply's and DF marine ply's, except the marine has a higher standard about void's. The marine has basicly no void's.

There are other maine plywood's. [Okoume & Meranti]

I personally would not use them as a transom core. They are more expensive and have better/smoother outer ply's.
They would be a excellent choice if you were building a whole wooden hull.

The transom core on the boat's we have is highly imbeded in fiberglass on both side's, so that super looking surface on Okoume is $130 a sheet & Meranti is $95 a sheet mean's nothing. A/B DF Marine here is $74 a sheet.
Any of these plywood's need extra attention to the end grain and any holes you bore thru it.