Professional mariners hold a variety of professions and ranks which are fairly standard, with the exception of slight naming differences around the world. Common categories by department include the Deck department, the Engineering department, and the Steward's department. Mariners can also be categorized by status as a senior licensed mariners or unlicensed mariners.

A number of professional mariners have left the industry and led noteworthy lives in the naval services or on the shore. For example, Traian Băsescu, started his career as a third mate in 1976 and is now the President of Romania. Arthur Phillip joined the Merchant Navy in 1751 and 37 years later founded Sydney, Australia. Merchant mariner Douglass North went from seaman to navigator to win the 1993 Nobel Prize in Economics.

Professional mariners

Seafarers hold a variety of professions and ranks, each of which carry unique responsibilities which are integral to the successful operation of an ocean-going 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.

Deck department

A common deck crew for a ship includes:

• (1) Chief Officer/Chief Mate
• (1) Second Officer /Second Mate
• (1) Third Officer / Third Mate
• (1) Boatswain
• (2-6) Able Seamen
• (0-2) Ordinary Seamen

Engineering department

A ship's engineering department consists of the members of a ship's crew that operates and maintains the propulsion and other systems onboard the vessel. Marine engineering staff also deal with the "hotel" facilities onboard, notably the sewage, lighting, air conditioning and water systems. Engineering staff manage bulk fuel transfers and require training in firefighting and first aid. Additional duties include maintaining the ship's boats and performing other nautical tasks. Engineers play a key role in cargo loading/discharging gear and safety systems, though the specific cargo discharge function remains the responsibility of deck officers and deck workers.

A common engineering crew for a ship includes:

• (1) Chief Engineer
• (1) Second Engineer / First Assistant Engineer
• (1) Third Engineer / Second Assistant Engineer
• (1-2) Fourth Engineer / Third Assistant Engineer
• (0-2) Fifth Engineer / Junior Engineer
• (1-3) Oiler (unlicensed qualified rating)
• (0-3) Greaser/s (unlicensed qualified rating)
• (1-5) Entry-level rating (such as Wiper (occupation), Utilityman, etc)