A ship's complement can be divided into four categories: the deck department, the engineering department, the service department, and other.

The main work of the deck department is proper watchstanding and the maintenance of the ship's hull, cargo gear, and accommodations as well as the ship's life saving and firefighting appliances.

Depending on the size and employment of the ship, a boatswain may be employed. If carried, the boatswain, generally a senior able seaman will act as a foreman of the ship's deck crew and as the chief mate's representative on deck, effectively as a fourth mate.

The Chief Mate is the head of the deck department. This involves administrative tasks such as scheduling work, quality control, coordinating with other departments, and conflict resolution. The chief mate also compiles supply, overtime, and cost control records, and requisitions or purchases stores and equipment.

Due to IMO regulations, larger cargo and passenger ships generally carry at least three able seamen and may carry ordinary seamen.

The ship's other deck officers, generally a Second Mate and Third Mate are also members of the deck department.

The engine department or engineering department is an organizational unit aboard a ship that is responsible for the operating the propulsion systems and the support systems for crew, passengers and cargo. This work is carried out by marine engineering officers who usually train via cadet ships sponsored by a variety of maritime organizations. Other shipboard departments typically include the deck department and thesteward's department.

At sea, the Chief Engineeris a licensed mariner who is responsible for all operations and maintenance that has to do with any and all engineering equipment throughout the entire ship.

The Chief's primary assistant, the 2nd Engineer, supervises the daily operation of the engine room and engine department and reports directly to the Chief.

The 3rd Engineer is usually in charge of boilers, fuel, auxiliary engines, condensate and feed systems, and is the third most senior marine engineer on board. Depending on usage, the 3rd Engineer is also typically in charge of fueling

The 4th Engineer is the most junior marine engineer of the ship. He is usually responsible for electrical, sewage treatment, lube oil, bilge, and oily water separation systems.

On a merchant ship, an oiler is an unlicensed member of the engineering department. The position is one of the most junior crewmembers in the engine room of a ship. The oiler is senior only to a wiper.

A wiper is the most junior crewmember in the engine room of a ship. Their role consists of cleaning the engine spaces and machinery, and assisting the engineers as directed.

The service department responsibility of living and catering services for the crew and other people on board. The service department is headed by a chief steward at the top and helped by stewards, bakers and mess men.