Why Are 96M Black Balls on This Reservoir?

The guy sounds very positive that they are safe. Too positive if you ask me.

That question occurred to me too. They say they last 10 years? Fantastic, that's very reasonable. What happens then? They dredge them all back out of the water and replace them? Because that sounds like a hell of a logistical problem compared to dumping them in.

Your link, however, appears to persist in the misconception that their purpose is to prevent evaporation. Why is it such a common meme?

I can tell you didn't watch the video

The video explains that the evaporation reduction was a side effect. The real reason is to reduce the level of cancerous Bromide down to safe levels, which could only reasonably be achieved by blocking sunlight from the water.

They tried a bunch of ideas for blocking the sun and settled on the balls as the best compromise between cost, effectiveness, and safety.

There are a couple of other benefits. They need far less chlorine to manage algae growth and it discourages waterfowl from taking up residence in the pool, also increasing water quality. They expect the balls to pay for about half of their cost over an expected 10 year lifetime from the savings in chlorine and evaporation.