Yes, it is. It is ridiculous on its face. Just try to justify a law against riding a manatee. By all means.
Even on "endangered species" grounds, please explain how playing with a manatee involves its death and / or inability to reproduce.
Guidelines
Follow these basic guidelines when you are near manatees.
General
Don't feed manatees or give them water. If manatees become accustomed to being around people, they can alter their behavior in the wild, perhaps causing them to lose their natural fear of boats and humans, and this may make them more susceptible to harm. Passive observation is the best way to interact with manatees and all wildlife.
Do not pursue or chase a manatee while you are swimming, snorkeling, diving or operating a boat.
If a manatee avoids you, you should avoid it.
Don't isolate or single out an individual manatee from its group, and don't separate a cow from her calf.
Touching
Look, but don't touch manatees.
Never poke, prod or stab a manatee with your hands, feet or any object.
Don't attempt to snag, hook, grab, hold, pinch or ride a manatee.
FPL | Basic Guidelines Regarding Manatees
Basic dos and don'ts
Background
The Florida manatee is an endangered species and is protected by state and federal law.
Don't harass manatees
Please avoid harassing or disturbing manatees. Harassment is defined as: any activity that alters the animal's natural behavior. By altering the manatee's natural behavior, you may create the likelihood of danger that is bad for the animal and against the law.
FPL | Protecting Manatees
And that is why people aren't allowed to touch/ride manatees.