A shipmate is literally a mate on one's own ship (i.e., a member of the same ship).
Typically, in the United States and other countries the term 'shipmate' is used amongst sailors as a generic name for those who are not on a name-to-name basis with each other. Because many ships have hundreds, if not thousands, of crew members aboard; shipmate is a universally acceptable name for all crew members. Within the United States Navy and the United States Coast Guard it serves as a convenient and respectful way to address other crew members when rank or naval rating is not immediately obvious. It also serves as a more professional expression in lieu of vernacular words.
In the United States Navy, one earns the name of a shipmate when he or she graduates from Recruit Training Command (or 'boot camp'). Prior to graduation from boot camp, only the terms 'recruit' or 'shipwreck' are used - which are considered for the most part derogatory. Earning the term 'shipmate' is a great honor for all new sailors in the U.S. Navy.
Conversely, in the United States Navy, the term "shipmate" can be the derogatory term used by officers and senior enlisted to refer to a sailor whose name they have not bothered to remember or read off of their uniform (Example: Hey! Shipmate! I need you to swab this deck now!). The term is not endearing when used in this manner.
Herman Melville's Moby-Dick, a popular maritime novel, is laced with the term, although the narrator Ishmael seldom uses the word: "This man interested me at once; and since the sea-gods had ordained that he should soon become my shipmate (though but a sleeping partner one, so far as this narrative is concerned), I will here venture upon a little description of him." [1]
One might refer to a fellow crew member by saying, "He and I were shipmates before reporting for duty here in Norfolk." The word is used in this sense in the old song "Don't Forget Your Old Shipmates".
When getting the attention of a fellow sailor, one might simply call out "Shipmate!" or "Hey, shipmate!"
When speaking to a group or crowd of sailors, i.e. "My fellow shipmates..."