Robert,
I admit I don't know a lot about the Sanibel area, having visited only once.
But I got the impression that the diving isn't stellar*. The water in those
parks is pretty murky and the vis is none too good. But I have vague memories
of a dive shop in the Venice/Osprey area that did shore dives for fossil
collecting, sharks teeth mostly.
Ellen
* And all things are relative. Compared to Connecticut diving, Gulf diving
must be dreamy.