covered with table tennis balls to keep in the heat.
I don't understand. TT balls INCREASE the effective area of the water, being continuously wetted over the whole of their surface area, thus increasing evaporation and also the direct transfer of heat to the air. Surely a load of balls?
Why would they be continually wetted? That would only occur I if they were moving, being hollow they would be trapped air and a good insulator. Depends how many layers deep too