If you haven't done so already, use Google to search for "all inclusive caribbean resorts for kids".

The Beaches resorts are the family-friendly extension of the Sandals Resorts all-inclusive chain. Both have been around for quite a while. They're a great (and safe) way to experience the Caribbean for the first time.

If your "mature kid" is really interested in the underwater world you might encourage him to learn how to scuba dive. It's a great way to learn new skills, build confidence, and interact with the ocean. For a first-taste of diving (perhaps for both of you) consider a 1-day Resort Course offered at your destination. Here's an example.

Turks & Caicos islands are also destination diving locations, so are probably your best bet for high-quality diving & snorkeling. The Jamaican resorts will probably be easier to get to. (You didn't say where you live.)

Here are a couple of links for reference:
Beaches Resorts- Turks & Caicos
http://www.travelandleisure.com/arti...ibbean-resorts