A reflective cover might not have the intended effect. From what I read (here: http://www.energysavers.gov/your_hom.../mytopic=13140 ) solar covers work primarily by preventing evaporation. I'm pretty sure any cover would reduce evaporation regardless of it's reflectivity. I guess it could work if you put it on in the day and removed it at night maybe?

There are water chillers on the market but they're expensive: http://www.google.com/search?q=swimm...+water+chiller

I remember asking this same question last summer maybe August. So far my water's pleasantly warm at about 90f in the evening, but it's sure to get warmer. I never did anything about it. Someone suggested maybe running the water through my heater with the heat off at night or something like that.