This page will have a guide to the treasure map you find in Tortuga in Assassin's Creed IV Black Flag
Treasure Map Location[edit]
The treasure map itself can be found in a Cadaver near the shore at coordinates (882,377). See map below:
Treasure Map Tool[edit]
This is the treasure map you find:
Coordinates indicate (333,650).
Note that the treasure chest is NOT located in Tortuga (where you got the treasure map from), but rather in Matanzas:
Dig Site Location[edit]
Dig location is around coordinates (342,642):
Located in a small cave near a stone pillar:
You get 4000 Spanish Reales and plan for elite mortar storage