You go to Abaco Island early in sequence 3
Treasure Map Location
The treasure map itself can be found in a Cadaver that's leaning on a tree at the southern shore of the island:
Treasure Map Tool
This is the treasure map you find:
Coordinates indicate (749,625).
Note that the treasure chest itself is NOT located on Abaco Island (where you got the treasure map from), but rather on a small island around 3000m to the southeast at the coordinates listed on the map:
This island is called "Salt Lagoon"
Dig Site Location
Sail to Salt Lagoon. It is recommended you first unlock the viewpoint on the southeastern corner of the island. Then, go to the northwest part of the island, see below for custom marker:
Find the pile of dry bushes on the ground, see screenshot below:
Dig there. If you do not have the option to dig just walk around until that option appears on your HUD.
Dig the treasure out you should get 3,000 Reales and plans for a serpent figurehead: