The mariner's coin is an item in the Temphere game universe.
A mariner's coin is given to someone who does a tremendous service to those who work on or live by the sea. It is a sign to other seafarers that the owner of the coin is a friend and that they are owned favors. Not all seafarers will necessarily care about or even recognize the mariner's coin, but it can be useful to have on hand.
In meta, it grants a +1 to charisma based skill checks when dealing with sailors.
After Fracture, McGuiggan, Athelia and Jolene liberated the Old Skyhorn Lighthouse from the control of the kuo-toa, the lantern keeper Lucien wanted to show them his gratitude for their help. He gave each of his rescuers a mariner's coin, explaining to them that it was a sign to sailors that the person who held the coin was owed a favor. He cautioned that not everyone might recognize what the coin meant, and some probably wouldn't care, but that it could help them out in a pinch.