A swimming pool in Florida without a heater will typically be the same temperature as the Gulf of Mexico year round.  A screened in pool will be 3 - 5°F colder.  (The Gulf temperature can be found on the evening news every night and also in the morning paper near the tides.)  A solar pool heating system, when properly sized and installed will give you an increase in temperature between 8 -12°F.  Typically this means that the temperature of a pool with a solar pool heating system will be above 80°F about 10 months of the year. 

Keep in mind that during January and February the Gulf temperature will stay between 55 and 65°F therefore, an increase of 10 degrees from the solar will put your non-screened pool around 65 - 75°F which will still be too cold to swim for most people.

A solar pool heating system, being dependant on the weather, cannot give you a 90 degree pool when the outside air temperature doesn’t surpass 70 degrees. 

Heating a pool without the use of a cover is much like trying to heat a house without a roof.  Your solar will put in a couple of degrees each day when we have warm weather and that heat will be lost whenever the evening air temperatures fall below 65°F.  It is necessary to leave the cover on the pool when the pool is not in use whenever the nighttime ambient air temperatures fall below 65°F. 

Without the cover each evening, all heat gained that day will dissipate through evaporative heat loss and give the pool a false appearance of not heating when actually it is heating but it is not retaining the heat.