How long will you be in Florence for? Because currently I think the most convenient as in that you don’t need to pay an “association” fee for is the Costolina right behind the Mandela Forum. That’s where I go to swim laps right now. You can either buy a pass for 10 entries and save like 5€ off the total cost of the 10 entrances or just pay a little more and pay by the entrance each time you go. Let me look for the website where you can see times and fees.