The bottom 8′ of the raked deck becomes a pool in Act 3. The TD is going to line the deck with pool liner and then cover that with a liner cover material called Rock On a Roll. It is a waterproof textured/painted liner used for homemade streams and ponds. It has a nice texture and paint treatment.
The designer wants to have the deck covered with sand. I have done this before with dirt, gluing the dirt with a trowelled clear glue called Super 88, but it is not strong enough to be submersed in water.
I do not think just piling the sand on the deck would work, it would never dry for two-show days.
I have a sample drying with Jaxsan that will test tomorrow. I know an epoxy material would work, Polygem has a few options. But the SDS states if not enough ventilation supplied air is required, so I am not in favor of that option.