To book a room in Key West during Fantasyfest it is a good idea to start a year in advance...Middle Keys are the best you will do at this time. Diving is better so don't get bummed yet...
Lodging ranges from the cheapest of roadside motels at $40.00 to renting your own private island for tens of thousands. Moderately and diver friendly is Holiday Inn in Marathon with a Marina and Abyss Dive Center on property. They run fast 6 packs and get good reviews. Diving will be @ 68-70.00 2 tanks. Rooms will be 70.00 to 80.00 booked through the dive shop.
If any of your group are willing to brave hanging out together in a villa or condo you can sleep 6 and get a kitchen for cooking for 85.00 and up.
Based on double occupancy and 2 boat trips and 2 tanks it will cost @ 980.00 per person not including air, transportation to the Keys, food, tips, souveniers etc...
If you rent the island it will cost $12,789.00 for the week + diving.