Last year we left Sept. 21, year before that it was Oct. 24, both times weather was fantastic. More than warm, downright hot on the shore, and at sea perfectly sublime.
The weather is warm until Nov., Dec. or Jan. September the weather is still very warm. Florida is still warm and the further south you go the warmer it gets. Winter in the tropics is different from winter up north. "The snow birds" tell me that there is an occasional cold day in January, but the "snow birds" (snow birds aka winter visitors) tell me that the winters in Florida are nothing compared to what they are used to.
Happy Sailing!