Mariner, sailor, seaman, tar, gob, bluejacket all denote a person engaged in sailing or handling a ship.
In nontechnical use mariner generally refers to those directly involved in the navigation and operation of the ship but in legal use it is applicable to a person employed aboard a ship in any capacity and then includes not only the navigators and operators but such persons as those concerned with the ship's business and housekeeping; thus, a ship's master, officers, engineers, and stewardesses all are in this sense mariners. Mariner is not so common as the other terms, but it is very common in literary use.
Sailor still so strongly retains its original implication of concern with the management of boats or ships that are propelled by sails that it is the appropriate term whenever this idea is specifically suggested. However the term is also applicable to a person engaged in the actual navigation or operation of a vessel regardless of the power which drives it.
In ordinary use it applies especially to one more technically called a seaman, one of the working force sometimes including or sometimes excluding officers employed on a ship. The term seaman alone is not ordinarily applied to apprentices, for it suggests skill and craft in operation and guidance of a vessel.
Tar is a familiar, often poetic, designation of a sailor; gob designates informally a sailor belonging to the navy and is not applied to an officer, whether commissioned or noncommissioned.
Bluejacket is commonly applied to an enlisted man in the British or American navy; the term originally referred to the distinguishing uniform of such a seaman; it is often employed in distinguishing a sailor in the navy from a marine or a sailor in the merchant marine.