Marine plywood, seal with epoxy.
While the floor is out go look at a bunch of boats in the showrooms, see if there's any floorplan features you'd like to add, take some pics on your phone, maybe carry a tape measure, and then:
New everything underneath. Hoses, wiring, pumps, fuel sending unit, and cables. Wire in a master power switch if you don't currently have one. Now is the time to even out the boat's balance as well. You can shift batteries and fuel load forward or aft depending on how the boat currently carries it's load. Run 2-3 extra sets of wires from the batteries to the helm and from the helm to the bow so you can add accessories by merely adding wire ends.
Now is also the time to upgrade underneath. Potential upgrades:
Bigger fuel tank if the current on has ever been an issue. More storage that's better though out, maybe even tackle and rod organizers cyphered in. Bigger/insulated live well. Recirculator pump, oxygenator, In-Floor cooler. Re-think seat base placement. Batteries and onboard chargers. Upgrade the fuse panel to use the little 2-prong plastic top fuses instead of those infernal glass pieces of shit. Upgrade switches. Add some LED lighting inside including the live well.
Before you add vinyl/carpet to your new floor pieces, lay it all in the boat and decide where to put a few hatches so that if you ever do need to get at pumps, wiring, etc, you don't have to hire a Romanian contortionist to do the work.
We replaced the floor in my dad's early 80's 16' Yar-Craft. Put the trolling batteries in the nose to hold it down a little. Took the 2 9-gal fuel tanks off the transom and put a 16-18gal tank in the floor dead center. It really improved planing speed as well as ride in rough water. Upgraded the switches and the live well. Cleaned up the old wiring and pumps, and increased the length of the rod box.
The idea phase can usually be accomplished with 4-6 beers and about 2hrs of just staring at the inside of your boat and mumbling a lot.