I believe I saw a bottle in the sand by the dock in a video somewhere, maybe on the AC website.
I think they should change the message in a bottle from random messages to treasure hunting quests. You could open it and find a full map or a piece of a map and when collected all pieces or the full map you could follow the map to the "X" and dig around to find a unique quest item or something. Or maybe you could find ancient treasure to put into the museum that are from a forgotten ancient culture :O