+1 for both. Grand Cayman I have taken my daughter. She was 15 when she went last. We dove with Divers Down right there in downtown Georgetown. The remains of the Cali (if memory serves) wreck is right in the harbour off their pier. Hardest part was helping her stand up with the tank on her back so we could walk her down the pier and throw her (giant stride) off the end. Easy dives.
Also +1 for Key Largo, however KL, at least the two years we have gone in January to get away from the Illinos cold, the water conditions (surface current and underwater current) can be unpredictable. This past January three of the five days (Duane snd Spiegel), dives were cancelled due to water being too rough. The two remaining days we hung out on Molasses reef, but even then, the afternoon dives of the 2nd day got cancelled. I had to get a tow into the boat after getting caught up and pulled away at the surface and one of the other ladies was knocked off the ladder coming back on when a 5' wave came randomly blasting through. Ive heard its smooth as glass other months though. Just my luck i suppose. Weve stayed at Amy Slates Amoray. Basic accomodations, but you Literally walk out of your apartment or room and step onto their boat. No messing about.