The only drawback I've seen to "plastic lumber" is that it is softer than wood. it bends, flexes, and conventional fasteners may pull out since it is often more like pressboard than lumber.
An inch thick also means twice the original thickness, a little headroom lost, and maybe some extra weight in the boat. You might wind up with a better result by using wood, and epoxy sealing it before installing it.
Then there's the question of holding smells: Sealed wood won't. But if you can keep ahead of leaks and keep the boat ventilated properly (solar vents have no equal) there's no reason you should have stinks on the boat.