If you want an energy-efficient way to heat your swimming pool and enjoy swimming for as much of the year as possible then a heat pump is the best way to do it.
A heat pump uses a small amount of electricity to operate. Instead of heating with an electric heating element they collect heat from the outside air. Even when it is 50 degrees outside there is enough heat in the air to put in the pool even though it feels cold to us. In other words, you are just really moving existing heat around from the air to your water, this heat is removed from the air, concentrated with your compressor, and then transferred to the water.
The best way to size a heat pump is to multiply the surface area (avg Length x avg Width) by the desired tempurature rise multiplied by 12.