What’s a mail buoy?
It’s an, ahem, watch duty assigned to young sailors on their first tour at sea. They are told that mail cannot be delivered directly to the ship, so a helicopter will drop the bag of mail into the water somewhere in the vicinity of the ship. Its location will be marked by a flag affixed to the top of the buoy that keeps the mail bag from sinking. The young sailor’s job is to go up somewhere high on the deck and watch for the mail buoy.
He is usually assigned to this duty for a few hours before someone lets him in on the joke.
I do not know if this prank is still played; I was in the Navy in the early 1980s when I heard about it.