For what you're talking about, a cruise on one of the older (not largest available, latest edition) ships sounds good. Your friends could visit a few different islands, you & your spouse could do a dive expedition at a port stop or two, and everyone could have fun on board.
All-inclusives + good local diving + a good party scene + cheap is likely not going to happen.
An often-cited cheap scuba destination with all-inclusives is Cozumel; you can go online and check out Scuba Club Cozumel and Hotel Cozumel; be mindful to calculate with non-diver rates in mind.
I recently had a 1 week vacation at the Courtyard Marriot (not all-inclusive) diving 5 days with Rainbow Reef Dive Center in Key Largo. Some hotels in regions that don't assume you're diving are pretty cheap to put extra people in, dropping average cost of stay, and airfare to Fort Lauderdale from Nashville (rented an SUV & drove down) was fairly cheap.
You'd be not too awfully far from Miami for fun; be mindful Key Largo isn't much on big sandy beaches, though.
If I understand correctly, Fort Lauderdale, and for better dive conditions nearby Pompano Beach, might offer a good vacation.
People on the forum debate the 'all-inclusive vs. roam the town' thing, but I've got a question for you: Since all-inclusives tend to appeal to dive-a-holics who don't want foraging to distract from their diving, and the 2 of you plan to just dive a day or two and you want a place where 'everyone can cut loose,' is an all-inclusive really the best plan for you?
Richard.