Food/diving/accomodations less than $2500, per person, per week, I assume? If so, Dominica might work. Airfare will cost more but on FF miles you might get there - American Eagle flies in from San Juan,
PR.
Or for that matter, why not Puerto Rico itself? The Desecho island diving (off the west coast) is supposed to be superb, and run about $100 / day / person for 2 dives. That might have to go up like everything though given fuel costs. Plus Puerto Rico gives the opportunity for lots of other stuff than diving - bioluminescent bay snorkeling, the rainforest and karst caverns, Arecibo (if you are, or are marrying, a geek
) shopping and dining in Old San Juan, the forts of Old San Juan, etc.
You might work things backwards, since you're using FF miles: find out where the miles will take you, and go from there. For all I know you've got the miles to get to Egypt and dive the Red Sea (also reportedly very affordable, once you're there).