You will ultimately get this when you kill the Cultist named Asterion, of the Gods of the Aegean Sea branch. He is stationed on a ship in the water between Skyros and Andros islands. (at least he was for me) You might have other cultists from this branch available who may also be on ships.
You'll have to defeat his ship and any other ships near him. Then board his ship and defeat him in open combat. Confirm his death, then loot the chests and the ship will sink.
Someone please let us know if sinking his ship before boarding is a way to go also.