We have the chance of a gift trip to the Caribbean. Trip would be roughly 10 days, either a cruise or all-in resort (Club Med type). I know nothing about the Caribbean beyond the basics - bunch of spread out islands with significant differences in local culture and "amenities" - so would appreciate hearing from some with greater knowledge of the area. My son is very much into marine biology and wants to snorkel (he hasn't done it before, I have) and see as much aquatic life as possible.

Amenity priorities:

1. easy to get to
2. all-inclusive lodging/ship
3. mature kid (13 yr) friendly
4. good food, but don't need *****

Activity priorities:

1. snorkeling
2. relaxing
3. "swim with dolphins/rays/whatevers" program
4. low-key sports - swimming, kayaking, easy hikes
5. historic sites

Not needed: nightclubs, casinos, singles scene, high intensity sports

Where would you go? Which island, which resort? Or would you cruise?

Advice appreciated.