We winter in the Florida Keys and love it
In September it is still hot during the day time.
In my estimation the nicest months are October November, March and April.
Right now it is very hot during the day.
As far as those that complained about the traffic. Those of us in the know only travel through Fort Lauderdale and Miami from 10:00AM through 2:00Pm . And never head to the Keys on a Friday afternoon, or out of the Keys on a Sunday night.
Make sure you talk to the "hurricane gods" about your September trip!
Guy
PS: (3PM) we just got back from one of our very own secret sandy bottom swimming holes hidden in the mangroves. It is about 50 feet long and 25 feet wide with a saltwater creek coming in one end and going out the other. The water is "gin clear" and chest deep. it is just great to hang out in on a hot day. Today we used our paddle boards to get there and back, and other times we use our kayaks.
There are hundreds of places all along both sides of the keys like this