Message in a Bottle is a treasure-hunt related item.
It is the only method for obtaining Treasure Map Fragments.
Message in a Bottle can be fished from special Fishing Spots, or picked up from the sand along the shore in random locations.
Opening a Message in a Bottle gives the player 1x Bottle and 1x Treasure Map Fragment.
At least 2 inventory slots must be free in order to open one. If your inventory is full, pressing E will not have any effect.