I was there 3 years ago and stayed for 3 weeks - SENSATIONAL........
You don't need a guide - we rented kayaks and paddled across the river/bay
to the spring where the Manatees hang out. We did not swim with them but stayed in the kayak and saw then swim right under us in crystal clear waters.
Lots of things to do if you are outdoors types, An hour north is Cedar Key - funky little "town" with a great beach. Rainbow Park (?) - swim with the Alligators !!!!. Ride a bike along the railroad path, visit the "zoo" in Homasassa (?). Went to a few different boat ramps along the coast and kayaked around - Airboat rides, a few nice state parks in the area and Orlando is only 90 minutes away.
Gerry