World Seed: 107016
Load up this world and you should spawn on a jungle island. Follow the coast line left(south) and you should hit the end of the island. You should be able to see a wooden pillar sticking out of the water. Continue south into the ocean and towards the pillar. When you get to the ship just go under the deck and grab the treasure map in the chest. Be careful not to drown.
Follow the map and it should lead you back towards the spawn island. The x will be next to a blue coral thing. Go two left of the coral and dig down. You might want to watch the video to show the exact location. Open the chest and grab the eye of the sea. The achievement should be yours.Edit: For some people the treasure is not where it is in the video. If it is not check the coordinates 712, 49, 200 or 5 blocks to the left of the coral then three out to sea. Thanks x Mataeus x for the exact location.Edit 2: seems to be about a 5 block radius where the chest can spawn. Just check all around the x. It might not be in the same location as the video.