Urethane Foam Cores
Urethane foam is a fantastic core material for making flat or curved laminates. It's lightweight, cuts easily, it's is highly resistant to rot, and it's chemically compatible with the resins used with fiberglass.
Torres Sport Fishing boats in Florida (among many others) are made almost entirely out of urethane cored laminate. These boats are light and nimble yet big and strong enough to weather tropical storms and big seas.
Urethane is great for making complex shapes and flat panels, but it's not without its drawbacks. Because it is simply a rigid foam, it has poor compressive strength. For this reason alone, it's not suitable as a core material for transoms. It's not uncommon for a transom to have twenty or thirty through-bolts, so the core material must be rigid enough to withstand that much compressive force. Urethane foam, for all its benefits, just isn't that strong. You can easily crush it between your fingers. Urethane foam is also expensive and not always easy to come by.