Water actually is a pretty good absorber for solar heat, so to reduce the amount of heat absorbed all that's really needed is to provide a more reflective surface on the water to bounce the heat back at the sky.

All that's needed is a layer of ping-pong balls covering the pool. They should be light enough to let you "swim through" them, and should reflect enough more of the solar IR to reduce the water temperature quite nicely.

For even better performance, you could consider "aluminizing" them, either by getting the novelty shop "silver balls" or "golden balls" or by painting them with aluminum colored Rustoleum® primer.

Since the increase in wet surface exposed to the air might encourage growth of algae on your balls, you'd probably want to give them a wash occasionally to keep the reflectance up to snuff. They'd float in the washing machine and you'd have to run small batches, so I'd suggest running your balls in batches in a solution of Clorox® in your cement mixer occasionally; although you might want to experiment some to find a best method.

If you get too fancy with the colors of your balls, you might still want some shade (privacy) fencing, since once word got out about Bobert's technicolor balls your pool could become sort of a tourist attraction, and you probably wouldn't want a lot of people looking at your balls while you swim - although a few friends probably wouldn't be much of a distraction.

John