your kids are 18 y/o and 20 y/o and you don't want them running around an all inclusive beach resort? um, i think you need to try a different planet.
Akumal Beach Resort, Mexico - great diving and snorkeling, good beaches and Sea Turtles come up on the beach at night starting in June to lay eggs Anywhere in Roatan, Honduras - World's second largest reef, 50 ft off shore Jolly Beach - Antigua - lots of white sand but zero pretty fish Moon Palace - Playa Del Carmen
Couple in Jamaica