I'm renting a pool home the first week of may 08 - We're opting for pool heat but even in May, we went back & forth on the decision. But if we were going in Sept, I wouldn't have spent the extra bucks.
I guess it depends on what you are used to. It usually stays warm here in FL through the month of Sept so you should be OK. My kids who were born in FL would probably say it's cold!!