I decided to varnish all of the ply wood areas. This will prevent damage, which was already happening with surf trips, and stop the wood absorbing water. I bought a light warm coloured varnish. This varnish would help to blend the darker 6mm and lighter 9mm ply.
I varnished the kitchen overhead cupboard about 4 times, the kitchen area about 3 times, and the rest just once or twice.