This was suggested a while back by another, but now that we have oceans it should be considered and slightly altered. viewtopic.php?f=48&t=46964&hilit
Now that we have oceans, messages in a bottle would be pretty neat! But not as just standard junk that washes up. When a M. Bottle is thrown into the water rather than washing up in junk like other garbage, it should generate as it's own object distinguishing itself from beach junk. It could also spawn in the water like a Peculiar Flotsam.
This would make coastal and sea life a little more interesting.
The item itself could either a reworked wine bottle (not sure how that'd work), or it's own item.