PVC foams have almost exclusively replaced the urethane foams that we used to use to in boatbuilding as a structural core material.  Foam cores were more commonly used in hull construction only however, recent developments have produced some excellent PVC foams that can be successfully used in deck structures.   Better known brands include Airex Tm and Core-Cell Tm.  Manufactured in different densities, foam core can be used for most boat building applications. A number of manufacturers market PVC foam cores to the marine industry in sheet form and as with the balsa products, solid sheets or scrim backed block configurations are available.

   You should check to see which material is locally available.   Make sure it is a PVC foam and it is the correct density for your particular project.   If you are using foam core for decks, you should ensure that it is of a suitable type for that application.