If you’re lucky enough to have your own outdoor swimming pool but not quite lucky enough to have a heated outdoor pool you might be interested in these Solar-Powered Pool Heating Rings which provide an ecologically friendly way of raising the water temperature.
The solar-powered heating rings float on the top of the pool and amplify any sunlight into the water thus raising the temperature. Each ring has magnetic edges so that multiple rings can be used to cover as much area as desired. Unlike most solar-powered gadgets these also work at night (well sort of) by acting as a cover to keep the heat in.
You can buy the the ring heaters from Hammacher for $70 a pair, in order to raise the temperature 3-4 degrees you’ll need a 70% surface covering.