I took a boat through 96 million black plastic balls on the Los Angeles reservoir to find out why they're there. The first time I heard about shade balls the claim was they reduce evaporation. But it turns out this isn't the reason they were introduced.
The balls are made of high density polyethylene (HDPE) which is less dense than water so they float on the surface of the reservoir even if they break apart.
They are 10cm (4 inches) in diameter and contain about 210ml of water. So the main reason they are on the reservoir is to block sunlight from entering the water and triggering a chemical reaction that turns harmless bromide into carcinogenic bromate.
This effect occurs with prolonged exposure to bromate so regulators insist that levels be kept below 10 microgram per liter on average over a 12 month period.