In seven years of full time cruising up and down the islands from the Virgins to
Grenada, we've never had a theft problem, although we have known people who were victims of petty thieves. As someone else mentioned, we avoid St. Vincent's
west coast anchorages, but otherwise we wouldn't say there is a theft problem, expecially compared to
Miami, which you mentioned.
Anchoring is free in the islands, with a few harbors being exceptions. Moorings are found in some places, but the quality varies. Buy good ground tackle instead of paying for moorings.
I'm not clear on whether you are planning to leave the
boat for 3 or 4 months, and if you are leaving it, would that be during hurricane season? If so, that narrows the posibilities somewhat. Leaving the boat
on the hard during hurricane season is possible, but not cheap, and requires advance planning. Most of the good yards for hurricane
storage are booked many months in advance. Leaving the boat in the
water for hurricane season is possible, but you need to have someone to look out for it. There are a few good hurricane holes, but I wouldn't leave the boat in any of them long term without someone watching it, especially if a storm is threatening.
More detail on your plans would get more specific answers.