I have been to all of these places except the Turks and Caicos. We loved Belize - if you are chartering out of Placencia you will have isolated anchorages, spectacular snorkelling and diving and not much in the way of other boats. Further north it is different I am told, but I have no experience of that. Navigation is by eyeball but not too hard if you keep a good lookout and get the cruising guide.
The Bahamas are very American compared to the other places and also quite shallow. Also really good snorkelling and diving and much easier provisioning than the other places. I liked it there but it felt like Florida except they drive on the left.
The Grenadines are spectacular - much more like ocean sailing but really worth it. St. Vincent was not a place I felt very comfortable but it is OK as a place to land at the airport, go straight to the charter bases (which are very close) and sail off right away. Bequia (a really nice place) is only a couple of hours sail away and everywhere else is south of there. But if you have crew who dont like high winds and swells then think twice about this, esp. in January and February when the christmas winds are happening.