Here is the long explanation: http://donnabarstow.com/park_blog/2008/04/14/dwp-dumps-hdpe-plastic-in-silver-lake-drinking-water/ but the short one is: plastic balls made of HDPE plastic that isn’t recycled, and most likely won’t be recycled that float on the water and repel birds.

They may help to prevent bromate forming in the water, because it sometimes forms when sunlight combines with chlorine.