I actually checked this out for a BVI charter I'm skippering in June. The speed for Internet access with T-Mobile is a fraction of what it is stateside. My experience with the area tells me coverage will be spotty as well, but I have no first hand knowledge of that.
T-Mobile has different programs for the access. The one I'm looking at will cost $25 for one month. They do limit your download capacity depending on the plan, and if you go over, they nail you big time with additional charges. You can cancel any time. By the way, I have T-Mobile's business plan, so a consumer plan might be different.
You should call T-Mobile. They are very helpful and will take the time to go into the plans for you.
No experience with phone data plans down here, but we've been able to get wifi on our computers and tablet almost everywhere from St. Thomas through the islands to Trinidad. A lot of places it's free, others it's us$40 to us$50.00 a month, or around us$10 a day.
Sorry it's off topic, but perhaps it will help.
Free international roaming with T-Mobile is 2G speeds (about 100k). It's fine for email and light data use (like reading TripAdvisor for dinner recommendations or looking up tomorrow's weather), but you won't be streaming video over it.