Bonjour et Bon Chance
Plan A:
Use these people
http://www.birdsunderwater.com/
For best chance of good photographs you must go out very early, before the noisy people show up.
Snorkle as quietly as possible. You don't need scuba.
The manatees are friendly and will usually approach. Let them approach you. They sometimes like to rub on your wetsuit !
You are not allowed to chase or harass them.
Water will be about 72d F. Wear at least a 3mm suit or more. Bring a jacket for the boat ride.
There is a an adequate motel (Best Western) & good restaurant (Charlie's Fish) next to Bird's.
Plan B: Get a room at Plantation Inn (nicer hotel) and rent one of their canoes or kayaks and paddle yourself to the Springs. (if you are good with boats & maps)
Plan C: try Homosassa Springs, Manatee Springs State Park(Chiefland), and Blue Spring (Orange City) State Park if conditions are unfavorable at Crystal River. Blue Spring is near Orlando.
As long as you are in the area, if time allows, arrange to snorkel, drift dive on scuba, or kayak the Rainbow River (Dunellon) or Silver River(Ocala). Both have crystal clear water and lots of wildlife.
PeterV, Florida