Message in a bottle is an item found right next to the southern dock exit on Goshima. It can be investigated to receive a treasure map, which leads players to Agar Atoll.
This message in a bottle was last visible 5 days ago on 2 July. It will be visible again in 5 days on 12 July. (wrong?) Once a message in a bottle is found and when a player has walked near its spawn location when it was visible it will always be visible, unless the message in the bottle or its treasure map is in the player's bank or inventory.
The location of the message in a bottle.