I think mr. arnold should save money to california tax payers by letting kids do that, instead of paying those workers high rates.

The side touching the water won't get very hot. Since they are sealed, there isn't much transfer of heat, and the balls will ride high in the water. Besides, the main purpose in having the balls isn't to prevent evaporation, it's to prevent a chemical change in the chlorine due to direct exposure to sunlight. Black spheres are more effective at absorbing light than white ones would be.

I thought the goal was to prevent evaporation

why not use white balls which will reflect sunlight rather than black which absorbs?

The balls are there to shield the water from sunlight which can react with chemicals in the water and produce carcenagens.

What is that?