Since you stated it came out in one piece, I'm going to assume this is an Aluminum boat. There's a brand call Arauco that's excellent, Also any plywood that's rated "Underlayment" is also good for transoms. Just make sure to Seal the edges and fill any voids to inhibit water penetration to the inner layers. Epoxy is the best resin to use for sealing plywood but there are several other good viable options.
