There's nothing wrong with a foam bulkhead if it is the right kind of foam (that doesn't absorb water). A closed cell PVC foam will work wonders and if you slab each side with a layer or two of fiberglass, you'll have modern bulkheads. Plywood is cheap, stronger, easy to acquire, but rots - it will rot especially fast if not COMPLETELY sealed. I would use some good closed cell foam to construct replacement bulkheads and fiberglass them in place.