They may use some desalinization in the Keys, where there are no rivers or streams to speak of, but it is more expensive to desalinize water than to process graywater or water from runoffs. I suppose that at some time in the future, solar power will be used to power some form of osmosis.
They are doing more of this in Saudi Arabia and the UAE than in Florida, because the water is scarcer, and the governemt gets more revenue from oil and such.