My wife and I found some deals for Cozumel and cheap air fare to various locations. For diving...check out areas devastated by the last Hurricane season. The British Virgin Islands had some specials recently. Try your basic websearch for a location and include "scuba or scuba diving" and you’ll get some good results.
Another option is call and negotiate a price with the dive shop/hotel combo you are looking at. If the time of the year is slow they will be willing to go down a bit on price (in some areas).
Try working directly with the dive shop/boat or hotel running the dives to get the best deals. Also, if you are willing to cook don’t overlook condos...we found a few who were cheaper than a hotel for a week long stay once we played hard ball on the price.
Good luck.