How to pick the best heater for your pool
There are basically three different forms of pool heat commonly used in Florida, heat pump, gas heater and solar.
Pool Heat Pumps
Warms the water in your pool without or without sun, and for a lower cost than natural gas or propane. A heat pump uses electricity to operate, drawing in warm air and converting it into warm water. Your existing pool pump circulates water through the heat pump to absorb the heat taken from the ambient outside air. The pump must be running to work and will NOT heat your pool in temperatures under 45 degrees f.
Gas Heaters are the quickest and the only method of heating that can guarantee year round swimming temperatures. The gas heater has about 3 x's the heat output of a heat pump resulting in quicker heating however this comes at a 20-30% higher maintaining energy costs.
If you're looking for some extra warmth and have clear southern facing space the size of your pool with 6 or more hours of sunlight a day then solar may be for you. After the initial costs of set up solar is a free heating option that has no emissions to boot. It typically adds 5-10 degrees to your water temp year round. It heats very slowly and could lose that heat if there is several days of obstructed sun.