I guess you gotta have a pool...or a really deep puddle...to use the ThermaSpring Solar Mat, but as long as that detail is sorted out, here's a natural, cost-effective way to raise its temperature from SwimWays. Place a series of mats over 70% to 80% of the water's surface to absorb warmth from the sun and reduce evaporation of heated upper layers, plus pool chemicals. Heat goes up, heating costs go down.
As the second half of their name suggests, ThermaSpring Solar Mats spring open and closed for portability and easy storage when not in use. The oval shape was designed specifically to fit all pool shapes and sizes.