King of Seas

You find sunken treasure through treasure maps, which you get off of fighting elite ships like Royal Navy Command Ships, Ghost Pirates and The Sovereign Pirates. The Royal Navy Command Ships seem to have the best drop rate for them. The best way to farm these ships is to take on side missions at ports that request you to fight and sink one.

Once you gave a treasure map, open up your map by pressing on your and tab over to your treasure maps tab. Match the islands you see on the treasure maps to the islands on your core map. It will give you a rough idea where the treasure is. You'll know exactly where to 'park' your ship by the golden ring that occasionally pings visually in the water.

NOTE: Floating wreckage and red X's on land do not count towards this achievement, although it seems you can rarely find treasure maps in floating wreckage.
NOTE: This achievement is stackable through multiple playthroughs.