The best time to get this map is early on sequence 4, since that's when the story lets takes you to Great Inagua and you can free roam there
Treasure Map Location
The treasure map itself can be found in a Cadaver inside a cave to the west of the docks. You have to do a tree run and then slide down a hole to reach the cave. See map below showing where the treasure is as well as where Edward is located for climbing the tree:
Treasure Map Tool
This is the treasure map you find:
Coordinates indicate (633,784).
Note that the treasure chest itself is NOT located on Great Inagua (where you got the treasure map from), but rather in Nassau
Dig Site Location
Fast travel to Nassau and set a custom marker at the location below (you can fast travel to the nearby viewpoint to make it faster):
The treasure is close to a big house, and in a swampy area. You have to drop down a little ledge of rocks. There is also a tree that appears to grow out of the swamp, diagonally.
See screenshot below to help you find the treasure chest:
Run around the base of that tree in the screenshot until you can see the option to dig
Your reward is 3,000 Reales and design plans for grey sails.