We convinced a friend and her son to try diving, so they completed PADI OW in Nassau.
As we began preparing for our first dive trip together as a group, she confided to me: "I'm not impressed. I don't get this 'diving" thing." To be sure, their diving to date has been pathetic; first some quarry diving and then OW in Nassau. Pretty lame, and not very pretty.
And it needs to be easy entry/exit for at least some older folks with knee issues, IIRC, plus you hate Cayman Airways and aren't convinced they're ready for drift diving in Cozumel. A big question is what would impress them.
In the Caribbean region, I was impressed with the coral reefs of Roatan (lush reef, but not as fish/big fishy as some alternatives; was based out of CocoView Resort), Little Cayman (it's my understanding Cayman Brac is similar albeit a bit deeper) and the outer atolls out of Belize via liveaboard. Belize rocked my trip, but in Little Cayman (via liveaboard) some of the wildlife came in closer, and we could literally touch a sizable Nassau grouper, so in terms of impressing newbies, Little Cayman might get the nod. I did some dives at Grand Cayman, not not enough to give a good run down on it.
But some people want 'big animals.' Then we're talking about the Goliath grouper aggregation out of Jupiter, Florida. If they're new to diving, I probably wouldn't push shark feed diving (oh, that can get interesting!). They may see some reef sharks. Or you could go in winter (you mentioned the December time frame) hoping to hit the lemon shark aggregation, and I suspect you may see some Goliath grouper incidentally, too. Jupiter is not a location for lush reefs with the underwater flower garden look, but it rocks in a different way.
At some point if you take a future trip a different time of year and want sizable sharks without shark feeding, the sand tiger sharks on the deep offshore wrecks out of Morehead City, North Carolina are worth a visit. That's deep diving with nitrox, fairly short dives, and also not a lush reef/underwater flower garden experience, but if you like big sharks and you cannot lie...
For some people, caves are interesting (but the thought scares me). Some people love nudibranchs and other macro photography subjects (but I doubt newbies have buoyancy nailed down for that). Some like historic wrecks. Back in 2013 I enjoyed Key Largo as a variety destination with a good mix of variously sized life, shallow and deep diving, reef and wrecks.
One thing that might be a swing vote - 'dive concierge' service (or lack thereof). IIRC, others reported at Little Cayman Beach Resort they do it for you. In my experiences in Jupiter and Morehead City, that was not the case...both areas catered to a substantial local (and more advanced) clientele (California was like this, too).
It's always possible diving just won't be somebody's thing. I love that Calvin & Hobbes comic strip where they're huddled in a tent, Calvin a suburban kid to the bone and his Dad the only camping-lover in the family, and outside in a downpour his Dad yells
"Hey, Calvin! You want to learn to gut a fish?"
Just got back from Bonaire where they are really trying to be "strict", and it didn't affect our trip in the slightest. They simply ask that, when you can, you rinse your gear in a tub with chlorinated water and try to dive sites that are in the same category. On a boat dive, they do all the thinkning on that for you.
