Material for training platform

[warhau]

My son is working on a Eagle project to repair a scuba training platform in the local lake. It has a nice steel frame, but half the boards are missing. It has/had grey, hollow 2x6s on it, and he's having trouble finding some suitable material to replace it.

Most decking isn't even supposed to be installed in contact with the ground, much less underwater. Was hoping shop staff or pros may have helped out with a build of a similar platform at some point, and be able to point us in the right direction.

Sorry if there's a better forum for this. I couldn't find one.

 

[Marie13]

I've only seen platforms that are either wood or metal. I've wondered, though, if any of the "plastic" composite materials that are used for backyard decks and the like would be suitable for this purpose.