This page will have a guide to the treasure map you find in Corozal in Assassin's Creed IV Black Flag
Treasure Map Location
The treasure map itself can be found in a Cadaver on a small piece of land across the northeast coast of the island, near a campfire, at coordinates (42,267):
Treasure Map Tool
This is the treasure map you find:
Coordinates indicate (55,178).
Note that the treasure chest itself is NOT located in Corozal (where you got the treasure map from), but rather in Ambergis Key
Dig Site Location
Note: You need the Diving Bell to get to the dig location.
Dig location is at coordinates (61,182)
You have to go through an underwater tunnel with urchin and Jellyfish
Here is the map of the destination:
Dig in the patch of ground near the wall at the coordinates (61,182):
You get 4000 Spanish Reales as a reward
