A solar cover, is one of the best ways to keep your pool water warm. 
 
The cover will assist in maintaining and increasing the temperature with heat from the sun, decreasing chemical costs by retaining the water under the cover and preventing water and chlorine from evaportaing or diluting. A solar cover also reduces teh need and stress on your heater, hopefully assiting in keeping hydro or gas expenses down.
 
Some covers will need to be cut to size to completely cover to the corners, edges and steps.