I've made this a sea cave to add interest, which means the sea is seen through an arch in the background.
Now to contrast with the gloomy cave interior I will inevitably paint the sea and sky a light colour.

But there's a problem as the Producer wants to open this scene in Ultra Violet light with UV painted sparkly bits. But light colours (especially blue) also glow under UV lights. So at the start of this scene we will have to blank out that light section.

Below is the completed 17 ft. wide cloth with a 2.5 Ltr paint tin for scale. Note I dropped the waterfall idea and replace it with a set of steps. As this was to be lit with UV lights on two occasions a waterfall would have been too light in colour. Seagull poo is a great way of showing the various planes in rocks! Hard to read in this jpeg but the writing carved in the rock says "Ben Gunn was here"

BLANKING CANVAS TO COVER "SEA"
The only thought I have at the moment is a "banner" of canvas supported by two poles. I would paint this canvas black and then paint a galleon in UV paint on it. This banner would be held by two people dressed all in black, and the canvas waved slightly to give the appearance that the boat is moving. Just before the UV lights are switched off the banner is crumpled up and dumped down at the back of the stage.

Update 14 Au gust..
This banner idea accepted by Sean and Robert but they want a skeleton (not as I had originally envisaged a ship) Apparently there is a dance by girls in skeleton costumes.

Note "My" skeleton will be nearly nine feet high!