re: ISO Aluminum plate ( Boat pictures added)

Personally I would lay foam between those stringers and then go with thin exterior plywood but paint it with deck sealer mixed with sand for nonslip or bedliner like wick said. Wont need thick stuff if you have foam for support. Nothing wrong with aluminum there though if you're set on it, a narrower boat wont experience as much hull flex and vibration. Plus you may be able to lay some weather stripping on the stringers to dampen the vibration.