They're Called Shade Balls
You may have read about municipal water facilities using floating plastic balls to protect water reservoirs and tanks from all sorts of things. There are a number of things that make these balls effective. I talked to one manufacturer of these balls. Keith Stuck of ECC, LLC makes shade balls (also known as algae balls and a bunch of different names). I asked him why they're being deployed in cities and counties all over the US. He told me that the balls are dumped into reservoirs or open storage tanks to act as a cover. The floating ball cover keeps birds out, eliminates some evaporation, keeps the water's temperature steady, and keeps a chemical called bromate from forming in the water. Because the balls keep the water temperature from rising, there's less algae to deal with.
There are so many uses that it would be impossible to cover them all here, but the main gist is that by adding a cover of floating balls to municipal water treatment reservoirs or tanks, the water is a bit safer. Water requires less chlorine and isn't fouled by birds that tend to want to hang out near water. Since the balls can last a good 10 years, and can be recycled after use, it's a win-win.
There's even a ton of videos on YouTube about it.