Is foam in a foam surfboard similar to one in a fiberglassed board?

They are usually made EPS foam, and polyester resin will melt it like acid. The blank is still usable, if you can work epoxy resins. 90% of the epoxy boards (especially the popouts) are made on EPS foam blanks.

Real surfboards are shaped from polyurethane foam and are then 'glassed' using fiberglass cloth and either clear epoxy resin or clear polyester resin. This is not something that you can buy in a boat shop. I don't know what you found but if the foam looks like one of the cheap throw away ice chests it is expanded polystyrene foam (Styrofoam) and is useless for surfboards. Almost any resin that uses a catalyst will melt Styrofoam. Even if you have found polyurethane foam the extra expense for cloth, laminating resin, finishing resin, tools, acetone to clean up with, fins and knowing how to attach them to the board, and the knowledge of how to build it should be enough to discourage anyone from trying to build one surfboard. You are talking about spending a lot of money on something that you could screw up in an instant.