Too cool for most natives to think about swimming, but Yankee snowbirds may think differently. Weather depends on frontal systems, a front comes thru, it rains for a day or two and then the temps drop for 2 or 3 days. Can't say how much they'll drop since it depends on the unpredictable frontal systems. We've already had two fronts come thru that dropped temps 20 deg below average with another one scheduled for this weekend. What that means for the rest of the fall and winter is anyone's guess but so far it is definitely been on the cool side since mid September. Meaning most days have been in the 80's or lower instead of the mid 90's.
If you want a better chance of warm weather then go further south to Sarasota and south on the west coast or West Palm south on the east. The Keys are the best bet but everyone also knows that, and it will be packed.