Maybe there could be a mermaid chest that you can hear from miles around if you face a certain direction. Like the other cursed chests. Other players can hear it too so it may be on an island or on another players ship. It won't stop until you cash it in of course.
But then you could also introduce mermaids that sing the same song, and so lure you towards what you think is the chest but it could ground you on some unmapped rocks. Occasionally.
