Since you posted this in the Lesser Antilles and mentioned St. John, I will say that in that area St. Croix could be good for your mix of people. Easy but good shore dives and snorkeling at the Frederiksted pier, snorkeling trips to Buck Island, good shore dives on the northern side and decent boat dives on the northern and western sides. We were there in March with a mixed group of 6 (2 divers, 4 non-divers, aged 16-75) and stayed in a condo at Gentle Winds -- not a bad location overall.
That said, if you aren't focused on that area and/or a U.S. territory, then I think Cozumel could be a good choice for your group also. Prices for lodging, diving, food, etc. are all much better, I believe the diving is better, food and service both definitely better, roads are better, things open later, etc. Just my opinion, but to me it's no contest if that is an option for your group.