Tom's Dive and Ski, Austin TX - Great group of people and also where I got my professional level certification as a Dive Master.
Yucatek Expeditions - Cozumel -- Really good place to get some more certifications. Completed NACD Cavern Diving Cert with German. Highly recommended. Very professional and personable.
Florida Cave / Cavern / Spring Diving Links
I found the following links in the November 2000 edition of Skin Diver magazine. The photography is impressive, not to mention the visibility!