A solar cover is essential and should be on the surface of the pool at all times times when you are not swimming.
A solar cover allows a build up of free heat from the sun during the day and it will also act like a blanket to keep heat loss at a minimum overnight.
There are many grades of solar cover, and generally the thicker the cover, the greater will be its heating properties and its lifespan.
Solar covers can also save money by helping to prevent pool chemicals from evaporating into the atmosphere.