I hate to be the voice in opposition, but bear with me. Dimensional lumber has the grain oriented in one direction providing for a stronger vertical strength. Plywood on the other hand is designed for horizontal, not vertical loading. This occurs from the various layers having differing grain orientations. Someone is likely to comment on parralams, or engineered lumber, but again, they have various layers, but the grain is oriented in one direction only. Now on a transom, your loading is designed for plywood to distribute the forces across a wider area. In wet conditions, I would prefer solid wood over a laminated piece. Consider this folks, which rotted out worse, your transom or your stringers? Now which one was likely to have moisture locked against it longer. And lastly, what did Glastron factory use? My $0.02, Michael.