Jamestown Distributors sells a number of different coring materials that have different physical properties to suit different applications. Out of these products, the one we recommend when redoing a boat's transom is Baltek Airex PXc Board. Being naturally water resistant and dimensionally stable, while boasting high shear and compression properties, it is an excellent replacement for balsa or plywood core transoms on boats.
The next best product that we stock is Core-Cell A500, which is available in double-cut sheets or plain sheets (plain sheets would be advised if possible for repairing a transom). This foam core material boats excellent dynamic loading properties and shear elongation properties.
Balsa core can also be used as an economically priced option. Balsa is one of the strongest wood species for its weight with high shear and compression strength, and is a renewable material.