A sailor, seaman, mariner, or seafarer is a person who works aboard a watercraft as part of its crew, and may work in any one of a number of different fields that are related to the operation and maintenance of a ship.

The deck department is an organisational team on board naval and merchant ships. The department and its manning requirements, including the responsibilities of each rank are regulated within the STCW Convention, applicable only to the merchant fleets of countries who have ratified it. The department is led by deck officers, who are licensed mariners, and they are commanded overall by the ship's captain. Seafarers in the deck department work a variety of jobs on a ship or vessel, but primarily they will carry out the navigation of a vessel from the bridge. However, they are usually also responsible for supervising and monitoring any maritime cargo on board, as well as ensuring maintenance of the deck and upper hull structure, monitoring the stability of the ship, including loading and discharging ballast water, carrying out mooring operations, and finally anchoring a ship.