This is a great question with no easy answer. Solar pool heating is a weather dependent technology that will make your pool warmer every day of the year. When conditions are better, especially over multiple days, solar pool heating will provide the best results. Of course, this is when you are most likely to want to swim, so that’s great news!
Properly sized and designed solar pool heaters in Southwest Florida will make your swimming pool approximately 10 degrees Fahrenheit above an unheated pool, and up to 15 degrees if you use a pool cover. Smaller bodies of water like inground spas can easily reach over 100ºF. Because unheated pools roughly follow the temperature of the Gulf of Mexico surface temperature, we can use this as a proxy for the temperature of an unheated pool.
Here is the most current Gulf of Mexico temperature in Naples, FL from the National Oceanic and Atmospheric Administration (NOAA):
Note: Subtract five hours from GMT to get local time in Southwest Florida (EST).
Source: NOAA, http://www.nodc.noaa.gov/dsdt/cwtg/egof.html
The Gulf temperature in Naples is very close to the temperature in Fort Myers, Port Charlotte, and all of Southwest Florida. Unfortunately, NOAA only maintains a station in Naples, FL.