My recommendation for dive shops are Crystal Lodge Dive Center,Adventure Dive center or Byrds Underwater. I rented boats from Crystal Lodge and have been on one trip with Adventure Dive in the Rainbow River. I got some very detailed solutions to a problem over the phone from Byrds. These are good places.
Regarding Dives:
The gulf is VERY SHALLOW here (Hernando & Citrus County). It is about 1.5' deep for every mile you travel offshore. Charter boats will cost $75-$125 depending on who you use and how far out they travel, and you probably wont get over 60' deep. Gulf diving is lower vis 5'-30' max and is great for spearfishing. Not so great for pictures or bright colored ornamental fishes.
If you go a little to the south in Pascso and Pinellas county it gets deeper offshore faster. I recommend Wet Yet Dive charters with Capt Earl in Pasco and Eastern Gulf enterprises with Capt Ty in Pinellas.
You can dive springs in the Homossassa River, Crystal River, and drift the Rainbow river. Other springs are Blue Grotto, Devils Den, Paradise Springs, Ginnie Springs, Manatee Springs. Some of these will have overhead environment so ask lots of questions before traveling there.
The best diving is a short trip across the state to the eastern shore. Jupiter, Boynton Bch, W Palm Bch, Ft Lauderale are all less than 5 hour drive and have much better vis. and lower boat charter rates. The ornamental fish are much more abundant there as well.
I hope this gives you some places to start looking. If you have any questions or need a buddy PM me and I will see what I can do help!
Safe Diving!