I re-built a marine rail system many years ago. Permits were not a problem then, but much more difficult now from what I hear. My buddy on Hood Canal was told no Army Corps permits are currently being issued for docks etc. because tribes are not approving them. May be a different story in other locations. Surplus rail is quite durable in salt water, mine are still useable 40 years later. If you can get a permit, guidelines will require that the rails are set on blocks above the surface of the beach. Much easier to do this if you are upgrading an existing rail system rather than starting from scratch.

Good luck.