Just my £0.02, but I would stick with marine ply.Although less common than it used to be and more expensive, it is still available and should be better quality.
The OEM floorboards have a combination of thin plywood for light weight, and mahogany edge sections and stringers to give stiffness and load spreading.
If you increase the thickness of the floorboards, you gain weight; if you reduce the thickness of the edge sections you reduce stiffness and also increase the point loading on the tube / floor seams.
I would be inclined to make one floorboard, but with the same overall dimensions, stiffeners etc as the original boards. I suspect size for size and weight for weight plywood is as good a material as any, without getting into exotics.
As Mart says, you should keep the front board separate so that it can flex.
There should be a fixed plank across the boat between the three back boards and the front board. Does your boat have this?
There are probably as many ways of finishing the boards as there are posters on Ribnet
. For simplicity and cheapness, a tin of International yacht varnish (the proper stuff from a chandlers not the B&Q equivalent) probably wins. For maximum performance (and cost!), a couple of coats of epoxy resin (to fully seal the boards) followed by a couple of coats of two pack polyurethane varnish (to provide UV resistance) will give you the best results, but is probably overkill.