What is "Hydro Foam"

I recently picked up a Yater that was built around 1990. It is glassed with epoxy resin and needs a few repairs. I would prefer to use polyester resin, since I have the materials and am familiar with the process. I can’t tell visually if the foam is polyurethane or polystyrene. Does anyone know what “Hydro Foam” is? Thanks, Nick