> there are parts of the Florida keys
Then allow me to ease your mind. Leeches are not a problem in the marine environment of the Florida Keys, unless you are a turtle. They person you replied to changed the topic slightly from the Everglades, where they could be a problem. In either case I'd worry about midges and mosquitos first.
Similarly with alligators, they are primarily freshwater and uncommon in the keys. American crocodiles can tolerate the marine environment better, but they are threatened as a species and have just two confirmed attacks in 75 years.
So wear a personal flotation device and you should be okay.