Precedented or not, that must have been a real treat to see! Some researcher somewhere would probably love to know about that, especially with such a good photo. Congrats!!
Manatees were removed from the endangered list earlier this year, finally. I have long suspected their numbers to be greater than what has been published, based on what my eyes tell me. They're like fleas on a dog around here these days. It would make sense if sightings further north became less rare in the future.
But they're always a treat to see. Kinda like dolphins, you never get tired of them. This guy actually chased us down recently (a very slow-motion chase, kind like OJ Simpson's) to interact with us: