A term used in todays Navy (US) to describe a well earned "day off" (for those not on duty). The ships usually come to a halt to allow swimming in the waters. Barbeque is abound and in general, good times. Other than the waiting in long lines (aboard an aircraft carrier) these few and far between steel beach picnics are welcomed by even the scurviest of dogs.
sailor #1- "That steel beach picnic was awesome!"
sailor #2- "Don't be an ass, you know some of us were on duty."