What type of all-inclusive are you looking for? A vacation club with some diving or are you looking for a dedicated dive resort? And when do you plan to travel? What is your budget?
If you are looking for a big all-inclusive then check out Beaches or Sandals Resorts and some of the other big AI resorts in the Caribbean. These resorts are often very luxurious and may also offer diving - although there are variable reports about the quality of the diving and the dive ops at some of these AI resorts so you will have to do some research. The Beaches Resort on Provo in the Turks and Caicos sits on the beautiful Grace Bay Beach and there is excellent diving in the T&Cs although I prefer liveaboard diving there because some of the best sites are rather far away.
There are also some AI resorts in Cozumel, Mexico that include diving and some of them are very popular with divers.
St. Lucia has several popular AI resorts and some folks love the diving but I was disappointed when we went there years ago. Pretty reefs but not many fish and a lot of small animals and sealife, but I haven't been there recently.
The Plaza Resort on Bonaire is an all-inclusive but there are many mixed reviews of the resort, food, and accommodations - so again be sure to do some research. To get the most out of a trip to Bonaire you should rent a car or truck and plan on doing a lot of shore diving.
If you are looking for a nice, small, diver-dedicated AI resort then I agree with
@bmorescuba and
@Doctorfish that you should check out the Cayman Islands.
The Cayman Brac Beach Resort and the Little Cayman Beach Resort and the Cobalt Coast Grand Cayman Resort are all owned by the same family. They are smallish, diver-dedicated resorts on the 3 Cayman islands with the same onsite dive op - Reef Divers - that provides valet services which is very nice and may make you feel like one of the Rich and Famous.
These are all small resorts (around 40 rooms) and CBBR is the nicest resort of the 3 IMO and it offers the best food and the diving is also very good. There is also a small spa on the resort property.
LCBR has the best diving in the Caymans because the Bloody Bay Wall and Jackson's Bite are among the best reefs in the Caribbean and it is a nice, small resort and the food is very good and they also have a small spa.
The CCGCR is located on the highly developed and popular island of Grand Cayman but they are located off the beaten path (but near the beautiful North Wall). The package includes 2-tank boat dives and unlimited shore diving on the House Reef but the shore dives are sporadic because they get winded-out fairly often.
I have never stayed at CCGCR but I have dived with their op and it seems like a nice place - but maybe not as nice as CBBR and LCBR, but that's my opinion and I haven't actually stayed or eaten there - just visited. I don't know if they have a spa but there are a lot of options on GC.
These 3 Cayman all-inclusive resorts are offering some great specials between mid-November through mid-January that include free Nitrox and free Nitrox certification if desired, see the first link below. The packages include 2 dives per day at LCBR and CBBR but you can upgrade to 3 for an additional charge. If you are not planning to travel during the holiday season, these resorts offer other packages throughout the year, see the second link below.
Free Diving, Nitrox, and Nitrox Certification*
Clearly Cayman
There are a lot of dive ops and places to stay on Grand Cayman but not many all-inclusives. You might also take a look at Sunset House near George Town (breakfast included but not an AI) and Compass Point on the East End, I don't think that it offers an all-inclusive option but it is a dedicated dive resort with apartments with full kitchens - but it is far from the most popular restaurants and attractions of Grand Cayman - but the East End diving is very good.
It's a big world of diving and soon you may also want to think about liveaboard diving, have fun!
