I need a lightweight thin waterproof panel?

I think I'd use plywood, exterior or marine grade, 1/4 inch or more depending on your weight. Not totally waterproof but won't rot easily especially if it comes out and gets dried out when you aren't using the boat. You can sand the edges off round, you can paint it if you want but if you do make sure you put some non-skid stuff on it. Those self-adhesive strips they sell for stairs would work nicely. Plastics pose the problem that if they're stiff, they can shatter into very sharp pieces. Wood can break too, but it doesn't break into knife edge pieces.