Gotta be the Plimsoll line.
A riddle! I run fore to aft on one side of a ship and aft to fore on the other. What am I? By Stuart O'Neill
I always wanted to hear the bosun announce over the box: "All hands aft, go forward. All hand forward, go aft. All hands midships, stand by to direct traffic". Alas, Bosun's Mates have no sence of wimsy.