This page will have a guide to the treasure map you find in Ambergris Key in Assassin's Creed IV Black Flag
Note: Visit the Tavern in Corozal to reveal the location of this treasure map.
Treasure Map Location
The treasure map itself can be found on a cadaver in the end of a hallway in Ambergris Key, at coordinates (58,184):
(note: Diving Bell required)
Treasure Map Tool
This is the treasure map you find:
Coordinates indicate (307,195).
Note that the treasure chest itself is NOT located in Ambergris Key (where you got the treasure map from), but rather in Misteriosa:
Dig Site Location
Set a custom marker here, at coordinates 301,202:
Locate the place with the water coming out of the stone statue's mouth and dig there. See screenshots below to help you find dig location:
Reward is 3,000 Reales and Elite Heavy Shot design plans