FRP Fiber reinforced panel would be my first thought. The panels are available in 4'x8' panels and are totally waterproof. I had them in my car wash for 8 years with no problems.
I cant remember what the price was, but they are not cheap, at least the version (white pebble finish) that I had. They are commonly used in commercial kitchens, medical facilities and farms.
Maybe this would give you some ideas.
PanelsThey have basically no structural strength, at least in the vervion I used, but there should be others which may work for you.
There are also some pvc based products.