Key points of World Map - Chests- Assassin's Creed IV: Black Flag
In the Caribbean, there are 46 chests with money. Most of them come separately, and there are only a few that contain the Letters in the Bottle or the Animus Fragments.
The chests can be found on the microscopic islands or on the sandy coasts of bigger landmasses. The difference between the landmass where there is a chest, and the one where there are none, should be easy to tell at first sight.
If you have not seized a fort with collectibles in the region, the game will automatically mark the chests on the map, at the moment at which you sail by.