I have done a series of my top three favorite Caribbean islands for scuba diving and my top pick is Bonaire.
It is 24 hour scuba diving, and everything is geared to the independent diver. The dive shops do not resent filling your air and it is inexpensive. The guide to dive sites is terrific. It is available everywhere and has a great description of over 100 dives. Many are accessible from shore. If you see a rock painted yellow with a name in black, that is the marker for a dive site. Pull over, enter the water. Have a great dive.
My close second favorite is Roatan and my close third favorite island for diving is Dominica. Dominica is the place to go if you want pristine diving as well as a totally undeveloped cool eco-lodge environment that has only been discovered by a few. Roatan has great diving as well as great beaches and a very laid back village with good restaurants and music and a great vibe.
Bonaire is where you go if ALL you want to do is dive. It is a scrubby island off the coast of Venezuela. Been Dutch, Spanish, English and is back to Dutch now. They speak English, Dutch, Spanish and their unique blend called Papiemento.
Not much to do but dive and eat at the great restaurants in the charming desert town of Kralendjick. Unless you love to windsurf, then go to Lac Bay. Windsurfers and divers and iguanas and flamingos. Huge natural congregation of pink flamingos. I kid you not. Oh and donkeys too.
The desert terrain means when you get those great tropical storms of the Caribbean, there is minimal dirt runoff so the sea remains clear. Great visibility almost all the time.
And you have heard that Bonaire has been a marine park for over 30 years so the diversity as well as the abundance of both fish and coral species is mind blowing.
You can stay in one of the big dive resorts, like Buddy Dive, for convenience or there are lots of apartments you can rent in town at all budget levels. I had a good experience with Sun Rentals. You will need to rent a car and about the ONLY car you can rent is a dive truck equipped with tank racks in the back. Perfect for this scrubby terrain with few roads.
Grocery stores are well stocked with foods you will recognize. Rent a car, get the dive guide book available everywhere. Pull over whereever you can enter the water and have a great dive. Pack a picnic so you can do multiple dives without anything but a quick stop overlooking the Caribbean Sea.
The dive shops will give you tons of free advice on where to go. And if you get a really calm day, go with a dive boat to Klein Bonaire. Translated to "little Bonaire", it is an undeveloped island less than one mile away with even more pristine diving, if that is possible.