I saw Kay's response to SVG. Good reefs around Tobago Cays and some good dive operations too. It can get busy around the Cays with all the day trippers and Yachts though.
Have you thought about Carriacou? Its the last (and largest) of the Grenadines chain of islands and is part of Grenada (with Petite Martinique). The direct translation of Carriacou from Carib Indian is 'land of (many) reefs' and this is a great way to describe our little gem. I moved here last year because of the diving and have been fortunate enough to see almost all that this island has to offer. Its small (13sq miles), off the beaten track, and tourism is not the main industry. The place is very friendly and peaceful and has a great community feel to it. There are three dive centres on the island and over 30 registered dive sites. Most of them are reef dives within the marine protected area. We're running a program with Caribsave (conservation charity) and our latest information shows that we have the highest level of fish biomass anywhere in the Eastern Caribbean, which is why we have such an abundance of amazing marine life, live coral and amazing diving.
Getting here is a short hop from Grenada and it is a totally chilled out and relaxed place. If you're looking for 5* resorts, this is not the place. If you're looking for warm, welcoming, relaxation with some great diving I would argue there are few better places in the Caribbean.
Gary