I have to say that a lot of times, the Lauderdale "sightings" by lifeguards and such are really tarpon. Ask a commercial fisherman.
Thousands of times I've been swimming there at all hours and crossed paths with dolphins (mammal type) sea cows, stepped on many rays and crabs, (scared the sh@# out on me) dozens of man of war (jellyfish) stings. The juvenile without the big blue bubble have less tentacles but still sting like a bitch, and put you off any more swimming.
They're out there. Many TV shark programs are filmed in the Bahamas. TV news helicopter shots of some shaddowy creatures are bound to be on tonight. The crusiers around here are MOSTLY non aggressive species.
Key Biscayne has a small history of bites.
Now you have another reason not to "pee in the sea".