Seafaring is a tradition that encompasses a variety of professions and ranks. Each of these roles carries unique responsibilities that are integral to the successful operation of a seafaring vessel. A ship's crew can generally be divided into four main categories: the deck department, the engineering department, the steward's department, and other. The reasoning behind this is that a ship's bridge, filled with sophisticated navigational equipment, requires skills differing from those used on deck operations – such as berthing, cargo and/or military devices – which in turn requires skills different from those used in a ship's engine room and propulsion, and so on.

The following is only a partial listing of professions and ranks. Ship operators have understandably employed a wide variety of positions, given the vast array of technologies, missions, and circumstances that ships have been subjected to over the years.

There are some notable trends in modern or twenty-first century seamanship. Usually, seafarers work on board a ship between three and six years. Afterwards, they are well prepared for working in the European maritime industry ashore. Generally, there are some differences between naval and civilian seafarers. One example is nationality on merchant vessels, which is usually diverse and not identical like on military craft. As a result, special cross-cultural training is required – especially with regard to a lingua franca. Another notable trend is that administrative work has increased considerably on board, partly as an effect of increased focus on safety and security. A study shows that due to this development certain skills are missing and some are desired, so that a new degree of flexibility and job sharing has arisen, as the workload of each crew member also increases.

The captain or master is the ship's highest responsible officer, acting on behalf of the ship's owner. Whether the captain is a member of the deck department or not is a matter of some controversy, and generally depends on the opinion of an individual captain. When a ship has a third mate, the captain does not stand watch.

The captain is responsible for the day-to-day affairs of the ship under their command. It is their responsibility to ensure that all the departments perform to the requirements. Therefore, the heads of the various departments answer to them. The captain represents the owner and hence is called "master."

Deck officers are licensed mariners who are responsible for the navigation and safe passage of the ship. Since the first half of the 20th century, the official ranks in the British Merchant Navy and others have usually been chief officer, second officer and third officer, although first mate, second mate and third mate are often used informally.

The chief mate is the head of the deck department on a merchant's vessel, second-in-command after the ship's master. The chief mate's primary responsibilities are the vessel's cargo operations, its stability, and supervising the deck crew. The mate is responsible for the safety and security of the ship, as well as the welfare of the crew on board. The chief mate typically stands the 4–8 navigation watch as officer-in-charge of the navigational watch (OICNW), directing the bridge team. Some crews have additional third mates, which allow the chief mate to not stand navigational watch, and focus more on cargo and deck operations. Additional duties include maintenance of the ship's hull, cargo gears, accommodations, life-saving appliances, and firefighting appliances. The chief mate also trains the crew and cadets on various aspects like safety, firefighting, search and rescue, and other contingencies. The chief officer assumes command of the whole ship in the absence or incapacitation of the master.

The second mate is a qualified OICNW watch stander, directing the bridge team and navigating the ship. The second mate is the third most-experienced deck department officer after the captain/master and chief mate. The second mate's primary duty is navigational, which includes updating charts and publications, keeping them current, making passage plans, and all aspects of ship navigation. The second mate's other duties may include directing line handlers, cargo watches, directing anchor detail and training and instructing crew members.