It's true that Grand Cayman is a sophisticated, developed resort destination but the sister islands of Cayman Brac and Little Cayman are small, rustic, quiet places with great local culture, excellent diving, and very little development.
Cayman Brac has only one hotel dive resort, the Cayman Brac Beach Resort. It is a small place with about 40 rooms and they recently renovated the property with a beautiful 2-level beach bar and a huge, freeform infinity pool. It is an all-inclusive, diver-dedicated resort, the food is served buffet style but it is very good. Reef Divers is the onsite dive-op and they offer valet diving as part of the service at no extra charge.
Brac also has some condos and a few other rental properties and restaurants and one other dive shop, the Brac Scuba Shack. Both of Brac's dive ops have excellent reputations.
We are heading back to the CBBR for the 4th time in November. They are currently offering an Autumn Escape Package that is $1325 US per person double occupancy. That covers your lodgings, all meals, diving, airport transfers, taxes, and service charges. It doesn't cover your air travel or dive shop tips.
The Autumn Escape only includes 2 dives per day but you can upgrade to add on an afternoon dive. They also usually offer 2 night dives a week. They have other packages with different options, see the link below:
Cayman Islands All Inclusive | Cayman Islands Vacations
Little Cayman Beach Resort is a sister property owned and managed by the same family. It is also a small, diver-dedicated, all-inclusive and Reef Divers is also the onsite dive op. The diving off Little Cayman is even better than Brac IMO but it a little more expensive and books up very quickly so it may be too late to get a reservation and find flights for this year. We were at LCBR last year and it was a wonderful trip, see the link below:
All Inclusive Caribbean Vacation Packages | Caribbean Vacation Packages
Both Cayman sister islands have nature preserves and seabird nesting areas and the local people are very friendly. You don't have to worry about drinking the water or eating the food in the Caymans and they have good healthcare facilities and mosquito-abatement programs.
Below are some links that you may find of interest. The water will be warm, low to mid 80s in the fall.
Good luck, finding a great dive vacation destination wherever you decide to go!
Cayman Brac:
http://caribjournal.com/2015/09/15/journey-to-cayman-brac/
Cayman Brac video:
https://vimeo.com/165357805
Little Cayman:
https://www.caymancompass.com/2015/07/24/From-Grand-to-Little--a-relaxing-island-hop/
The differences between the 3 Cayman Islands:
http://www.tripadvisor.com/ShowTopic-g147364-i260-k8593445-The_differences_for_a_diver_between_the_3_Cayman_islands-Cayman_Islands.html