I have been to TCI 3 times. The first time was shore based in Provo. The second was a LOB (Turks & Caicos Explorer II) and the third was a shore based in Grand Turk.
The three trips were very different. For the one where we stayed in Provo, we stayed at the Best Western (now it is called the Ports of Call) and dove with Dive Provo. This is a very convenient setup since they share the same parking lot. It is quite literally a matter of walking next door after breakfast. The draw-back, however, is as others have mentioned that the boat rides are 45 minutes give or take a little. The diving tends to be wall dives with the crest of the wall starting at about 45-60 feet. It does tend to be fairly "sharky" with Caribbean Reef Sharks and Nurse Sharks often coming over to say "Hi". Dive Provo is a class act, and I can recommend them without hesitation. (Count on 2 dives per day.)
Provo has a reputation which is much deserved for being expensive. There are ways to mitigate this. There is a grocery store about a 5 minute walk from Ports of Call with an amazing deli that has everything from pasta and pizza to sandwiches and salads.
The Turks & Caicos Explorer II (TCEx2) went to many of the same dive sights that we did with Dive Provo, but instead of a 45 minute boat ride, you simply step off the back of the boat. They might go a little farther from Provo, but not too much. (Count on 4 dives per day, 5 if you do the night dives.)
The third trip was to Grand Turk, and I stayed at a resort called Bohio. The diving in Grand Turk is similar to that near Provo, in that it is all wall diving. The big difference is that the dive sites are so close that you are already in your wetsuit before you get on the boat. Rides were typically less than 10 minutes (often less than 5 minutes). After the first dive, we would come back to the Bohio to place our lunch order during our surface interval. After the second dive, let the wait staff know you are back, go to your room & shower and get dressed and when you get back to the restaurant, your lunch order will be ready for you. (Count on 2 dives per day.) I personally had a lot of fun at Bohio and would go back tomorrow, but if you are looking for nightlife, it may not be a good fit for you. There is a
huge difference between the nightlife in Grand Turk and in Vegas.
One thing to keep in mind is the time of year that you go. If you happen to go in the spring (both of my shore based trips took place in March) the Humpback whales are migrating through the area. You will likely hear them on most dives if the timing is right.
OK, what to expect on the dives. First of all, from the folks that I have talked to, the two hurricanes that hit TCI last fall did a number on their infrastructure, but the dive sites are deep enough that there was virtually no damage to the dive sites.
The dive sites tend to be walls. Sheer drop off, pure vertical walls. Walls that drop off to the abyss (keep an eye on your depth). The coral was good and the reef life was abundant. Sharks are common and friendly, but not aggressive. There were a lot of turtles in Grand Turk. It was unusual if we didn't see at least one per dive, and sometimes 5 or more were seen. Lionfish were present, but no where near what is expected in the Bahamas, and they haven't wiped out the other species.
Personally, I really liked the TCI.