"For all you landlubbers out there" joke

Here's one people can tell to their grandmothers:
Once upon a time there was a famous sea captain. This captain was very
successful at what he did; for years he guided merchant ships all over the
world. Never did stormy seas or pirates get the best of him. He was
admired by his crew and fellow captains.
However, there was one thing different about this captain. Every morning
he went through a strange ritual. He would lock himself in his captain's
quarters and open a small safe. In the safe was an envelope with a piece
of paper inside. He would stare at the paper for a minute, then lock it
back up. After, he would go about his daily duties.
For years this went on, and his crew became very curious. Was it a
treasure map? Was it a letter from a long lost love? Everyone speculated
about the contents of the strange envelope.
One day the captain died at sea. After laying the captain's body to rest,
the first mate led the entire crew into the captains quarters. He opened
the safe, got the envolope, opened it and...
The first mate turned pale and showed the paper to the others. Four words
were on the paper, two on two lines:
''Port Left, Starboard Right''