Welcome to Marriott Rewards Insiders, vacay! I'm very excited to have you as part of our community, and am glad to see that you've already jumped in by asking a great question. Here is the Getting Started Guide, which has tons of information and tips for using the page that I think you'll find useful.
I, sadly, have never been to the Caribbean, but am interested to hear what other Insider's recommend. I've pasted some previous threads below, that may have some useful insights.
Thanks for posting!
Don't overlook British Virgin Islands Resorts | Scrub Island Resort | BVI Luxury Resort the place is absolutely fabulous. You can fly into San Juan and take a 45 minute flight to Beef Island (EIS) on Tortola, hop on the Scrub Island Ferry that runs every hour and be there in 10 minutes.
St. Maarten also is beautiful however; if you want to keep with Marriott properties there currently are none located on the island.
Let us know what you decide on and safe travels.