I would have to say St. Vincent ad the Grenadines - 32 islands, amazing sailing, take your breath away scenery and a very broad selection of diving
St. Vincent - well known as a critter diving location, but also has over 40 named dive sites plus much more unexplored in the north leeward coast. Huge drop offs, couple of very nice wrecks, caves and caverns and untouched reefs to explore. (Another benefit, it's off the beaten track, so usually you'll dive with groups of less than 4 others).
Bequia - Much beloved by the yachting crowd. Deserted white sand beaches, small selection of shops and restaurants selling tourist fare, and diving that offers sharks, larger pelagics, super drifts, and sparkling coves packed with so much fish and reef life it'll take your breath away!
Mustique - Home of the rich and famous, property owners read like a celebrity who's who! Prince William and his squeeze de jour visit on a regular basis, and when you take a drink at Basils Bar you never know who you'll bump into! (The last time I was there Sarah Jessica Parker was picking up a few things at the local bakery!). Diving around Mustique offers the chance to glide with Eagle and Sting Rays, and encounter sharks in and around the pillories dive site. Watch out for whales and dolphins too.
The Tobago Cays - These islands were used in the Pirates of the Caribbean movies. (Remember Captain Jack marooned on an island with no rum! This was the island of Petit Tabac). A marine park, the Tobago Cays provide divers with the opportunity to dive with hawksbill and green turtles, experience stunning drift dives such as Snake Passage where the currents have cut an underwater highway through the reef, or Mayreau Gardens. Or visit the wreck of the Purini WW1 wreck sitting in just 45 feet of water and swarming with thousands of fish!
Union Island - The last large island before heading south towards Grenada. Union offers great dives around it's coast line, and also offers access to the small uninhabited outlaying Grenadine Islands.
Other islands which offer diving and accommodation include Canouan (Raffle Resort and Casino), and the private island resorts of Palm Island and Petit St. Vincent.
St. Vincent and the Grenadines offers something for everybody, no matter the size of budget, or accommodation requirements. Each of the main islands have airports (St. Vincent and Canouan can be reached directly from St. Juan PR, and there are connecting flights from Barbados, Antigua, Grenada, St. Lucia and Trinidad).
Prices for a 7 night / 10 dive package start from around US$750 per person. Long term accommodation is available, and a lot of smaller family owned properties will offer long term lets. Although if you have 6 weeks then you could undertake a real island hopping experience!
Peter Hughes Wind Dancer is also operating between Grenada and St. Vincent traveling the Grenadines for the summer, so this may e an option to incorporate into your plans!
Hope that this helps!
St. Vincent and the Grenadines