The Mail Buoy
Pranks are an important element of military culture, just like they are in every other branch of the military. Submarines are no exception. The request to pick up the mail buoy is a hilarious newbie prank.
One of the submarine’s newest sailors is requested to put on foul weather gear and climb atop a mail sack supposedly attached to a buoy in order to retrieve some mail. Of course, the request is denied because there’s no such thing as a mail buoy.