There is another, similar hole farther north, 28 miles off Redfish Pass (between Captiva and Upper Captiva) that drops from about 80 ft - 170 ft.
Is that one known as Captiva Blue Hole?? (if so, I've gone diving there)
Lee County has published a list of artifical reefs that lie off out shores. Many of them are excellent dive sites.
Walter - nice list. How many of them do you know the depth on??