I did that 2 weeks ago with a black 5gal pail, and my wort chiller via Solar Gain. It was crazy hot and Sunny so I threw the black 5 gal bucket out there in the sun for a while and put my chiller in it. Slowly ran water into the pool. It didn't help much, but I've had that plan for a while now.
I actually wanted to build some solar heaters for people as a way to generate some cash flow to support my brewing habit. My part of the country doesn't have the Temps to heat up a good size pool consistantly. But we do have decently sunny days. So I figured a pond pump and black hose inside a black box could circulate and increase temperature over time.