Treasure Map Location
The treasure map itself can be found in a Cadaver at the northeast part of the island, at coordinates (335,335):
Treasure Map Tool
This is the treasure map you find:
Coordinates indicate (901,263).
Note that the treasure chest itself is NOT located on Cayman Sound (where you got the treasure map from), but rather in Petite Caverne
Dig Site Location
Go to Petite Caverne at coordinates (901,263)
The dig site is at the southwest edge of the cave, at coordinates (894,260)
Loot: The treasure chest contains money and plans for elite heavy shot storage.