This one has always been my favorite.

A Sailor is relieving himself in the head when a Marine walks in and steps up
to the urinal beside him.
After a few seconds the Sailor finishes, shakes, zips and walks over to the
sink to wash his hands. The Marine also finishes, zips-up, and walks to the
door.
Just then the Sailor says, Hey Marine! When I was in boot camp, they
taught us to wash our hands when we finished!
The Marine looks at the Sailor and says, When I was in boot camp, they told
me not to piss on my hands.