+1 Aluminum sheeting is good, composite (plastic decking material) is too. Plywood is probably the cheapest, easiest way to go. Just be sure that it is NOT pressure treated. Instead, use high quality, 1/2"-3/4", exterior grade plywood and seal it with several coats of Spar Urethane. You should be able to get both at the local hardware store. You could go all out and seal the wood with 2 part epoxy, or fiberglass and polyester resin, but that can be pricey and you often have to order the supplies. Then attach the floor to the boat using either aluminum, blind end pop-rivets or stainless steel sheet metal screws dipped in a bit of 3M 5200 marine caulk, which you can also get at most hardware stores these days.