What is the difference between the merchant navy and the (military) navy?

Confusing the two is understandable given the shared vocabulary, ranks like officer and master, terms like watchkeeping and seamanship, and the fact both operate large vessels at sea, but the purpose, employment structure, and legal framework of each are entirely separate, and the distinction matters practically for anyone choosing a career.

The merchant navy is not a single organisation at all; it is the collective name for a country's commercial shipping industry and the seafarers who crew it. A merchant officer is employed by a shipping company, or through a manning agent on that company's behalf, on a fixed-term seafarer employment agreement, moves between employers and vessel types over a career, and works toward internationally recognised certificates of competency issued under STCW, which are portable between flags and companies in a way that lets an officer build a career across many shipowners. The ships themselves, tankers, bulk carriers, container ships, cruise vessels, are commercial assets whose purpose is to earn freight or fares, and their crews are civilians subject to civil and maritime law, not military discipline.

A navy is a branch of a state's armed forces. Naval personnel are enlisted or commissioned into military service, subject to military law and command structure, trained in warfighting, weapons systems and combat operations alongside seamanship, and their vessels, warships, submarines, and support ships, exist to defend the state and project power rather than to carry paying cargo. Career progression, pay structure, pension and discipline in a navy run through a completely different system from the commercial one, and moving between the two, while not unheard of, means starting a genuinely new career track rather than a lateral move.

For a young person deciding between them, the practical differences are significant: merchant navy training leads to civilian certificates usable with any commercial employer worldwide, contracts are typically a period aboard followed by a period of leave, and the work is trade-driven, going wherever cargo needs moving. Naval service is a long-term commitment to the armed forces, with postings, deployments and a chain of command set by the state, and a different set of physical, security and citizenship requirements typically applies. Both demand competent seamanship, but they are answers to different questions: commerce versus defence.