Richard,
You can easily do 3 dives per day on Grand Turk. The wall (begins typically at 30 ft and drops to 7000 ft) is literally a 5 minute boat ride away from the beach. You are able to see the mooring balls from the beach. Because of the small numbers of divers, you are able to dive your computer limits and safe air limits . My dives averaged 70 minutes and I even had one 90 minute dive.
Bohio was very laid back. The 1st trip usually departed about 9:00AM and made it back to the beach at 11:00AM. You would do your surface interval and order lunch to be served when you arrived back after the second dive. If a 3rd dive was done, it would depart about 3PM. I did a couple night dives as well.
Three of my 7 dive days were two tank days. On the first two dive only day, we took an afternoon trip to free dive for conch. After collecting the conch, we went to a small cay and cleaned the conch for conch ceviche. While the ceviche was stewing, we fed the scraps to stingrays while drinking rum punch and soaking up the sun. Since there were two 50+ year olds (me included) the captain/conch diver put us in only 15-20 feet of water for the free dive.
On another two dive day, my wife and I rented a golf cart for the afternoon and explored the island. Much too see. Lot's to see. Birds, wild donkeys, wild horses, lighthouse, churches etc...
The other two dive day, we snorkeled on the beach, sailed a hobie cat and relaxed.
I to enjoy diving as much as possible when travelling. I understand the 4 dives per day and I normally do the same thing at Rainbow.
If you want to try the Turks and Caicos, the humpback whales migrate through the Salt Cay area (10 miles from Grand Turk) January through April. Salt Cay is suppose to be the best base point for this exploration . Our Dive Master at Bohio (Ollie) was filling in for Bohio's Vacationing Dive Master. Ollie has his own dive operation at Salt Cay and they do humpback snorkeling trips.
Dive Salt Cay Divers, scuba diving, Salt Cay, Turks Caicos Islands
Turks and Caicos Whale Watching - Caribbean Humpback Whales
One of our fellow divers at Bohio left TUE and went to Provo to dive. He told me that Grand Turk in general was a much better dive spot with the exception of diving West Caicos (long boat ride) where they encountered many black tip reef sharks).
If/when I go back, I would target the Humpback migration time. I would spend a day diving West Caicos, go to Salt Cay for several days and finish at Bohio on Grand Turk.
If I may answer any additional questions let me know.
David