From my hotel room, I saw a large dark shadow swimming down Fort Lauderdale beach today. I did my best to zoom and videotape it with no tripod. I was surprised how close it swam to people and no one seemed to notice. My best guess is a seal. Any others? I turned the sound off per Sasha's request.
Edit: Redditors suggest that this is likely a manatee, and that sounds right to me!