There are many treasures in the game. You will find these by discovering a 'treasure map' which you can gain for different locations.
So far I have found these random treasure maps when I took over a few lights houses, and when I sunk a few of the golden ships that are floating around (with the time limit for destroying them)
You will then be required to sail to them and use your spy glass to locate them.
I will note this achievement didn't pop in game, I had to exit later to see I got it (which I didn't do right away, it was after I had collected 4 treasures).
Happy Treasure hunting