Unheated and uncovered pool temperatures are closely approximated by the Gulf of Mexico temperature in Southwest Florida. If you want to see how your solar pool heating system is performing, compare the current temperature to the current Gulf temperature!
Here is the most current temperature in Naples, FL from the National Oceanic and Atmospheric Administration (NOAA):
Recent Water Temperature: 74.8°F (23.8°C)
Observation Date and Time: Wed, 01 Apr 2015 03:48:00 GMT
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. Details about this station can be found at the following link: