I have used both foam and ply for bulkheads. After a couple of years I have developed a preference for Ply or carbon covered ply.

EPS foam or Styrofoam is light, cheap, and very strong in compression. It also must be kept dry. It absorbs water and once it absorbs water, its tough to get rid of it. Some sailboarders actually built a giant centrifuge to dry flooded cores. EPS is hard to seal against water because it is soft and so many things dissolve it. Epoxy and the lightest fiberglass you can find works well. 2-3 inch (50mm-75mm) thick foam is about right

PVC or polyurethane structural foam like Jake was talking about will work. It will be heavier an and much more expensive but it will not absorb water 1-2 inch (25-50mm) thick structural foam not core foam is about right

Plywood is light, cheap and strong in all directions. Sealed with epoxy and kept away from UV, it will last a long time. 4mm plywood with medium weight carbon/kevlar vacuum bagged to both side is a hell of a structure.

I vote for plywood.