I have never experienced this before!
My kids and I checked out and returned the bedroom keys on time at 10 a.m., but we then decided to enjoy the swimming pool until noon before having lunch nearby and starting the 2½-hour drive back home.
At around 11 a.m., however, while we were the only ones in the swimming pool, the manager came over to us and, in a very strict tone, told us that he had been very kind to allow us to stay there for one hour after checking out and that he wanted us to leave the pool immediately.
I was quite surprised, especially by the tone. During previous stays at various hotels in Florida, we have often enjoyed using the hotel's swimming pool for a couple of hours after checking out, before leaving the area, and not once has anyone ever said anything to us. My understanding is that this is fairly common for guests on their last day, particularly when they have already checked out but are still spending part of the day in the area.
Needless to say, when we return to this area, I will make sure to avoid this hotel.
NOTE: The rope and floats around the pool are very old and brittle from prolonged exposure to the sun. They clearly have not been replaced for a very long time and, in my opinion, may no longer meet current regulatory requirements.