In the game, there are 22 hidden treasure coves. 19 of them can be found in the single player mode. The remaining 3 become available thanks to the Kenway's Fleet.
Each of the maps that can be obtained in the single player mode, has been marked on the world map as a "corpse" and it is held by a skeleton in its hand. Collecting the map will cause it to display, with the treasure coordinates.
If there is a treasure in a given area, to which you have the map, the game will notify you of that. Select the map on the toolbar and look for the one with the coordinates written down on it. Then, go to the location where the treasure is supposed to be. Take into consideration the locations of trees, boulders and the other characteristic elements. Take a look around the area until you find an interaction prompt on the ground.
In each treasure cove, there are regals. In some of them, you can also find decorative elements for the Jackdaw and design plans for upgrades.