for Manatees, Crystal River is the place to go. you'll want to go there
between mid November and mid February to see the most. You need a boat
to get to them, and unless you know where and when to look, your best
bet is to go with a guide. I recommend this guy (thanks to Ron T):
http://www.aquamarineimages.com/ Other people whose opinions I trust
recommend Bird's Underwater. If you want to rent a boat and try
yourself, I like the dive shop at the Best Western Crystal River Lodge.
They will give you some pointers on where to go.

One operator there is particularly bad: American Pro Divers.

There are a couple of spring dives at Crystal River in Kings Bay. One
can be accessed by walking down the Best Western boat ramp, the others
require a boat. The Rainbow river is close by, and it is a wonderful
shallow drfit dive or snorkel. you park in a park on the river and pay
an operator something like $10 to take you upriver and drop you. Devil's
Den and Blue Grotto are within a hour's drive. These are caverns that
allow OW divers to get a peek. A bit farther is Ginnie Springs. It has 1
cavern for OW divers, and some that are not suitable for divers without
cavern/cave training.

Hope this helps

al

Don't even think about sinkhole and cave diving without advanced training.