The propulsion plant of a vessel and management of the engineering department is the responsibility of the engineer officers. Ship engineers operate, maintain, and repair propulsion engines, boilers, generators, pumps, and other machinery. Merchant marine vessels usually have four engineering officers: chief engineer, a 1st assistant engineer, 2nd assistant engineer, and 3rd assistant engineer.
Assistant engineers stand periodic watches, overseeing the safe operation of engines and machinery. All engineers "stand" watch for specified periods, usually a 4 hours on/ 8 hours off rotation. Engineers supervise crew members engaged in maintenance and the primary up-keep of the engineering plant.
Engineers work primarily on commercial ships (merchant marine) or Military Sealift ships, in water transportation services (such as working on tugs, charter boats and other watercraft) and on vessels which carry passengers (such as cruise ships, casino boats, sightseeing boats, and ferries) Rotation schedules vary from 2 weeks up to 120 days, depending on vessel type. Employment outlook is favorable with competitive starting salaries.
Because of a their thorough knowledge of the design, operation, and maintenance of ship systems, sometimes referred to as a "floating city, marine engineers' career opportunities outside the shipping realm are plentiful and lucrative.
The U.S. Coast Guard promulgates the requirements for documentation and issuance of merchant mariner licenses for Deck and Engineer officers. Engineer officer licenses are issued by:

License advancement is based on documented sea time, professional recertifications, and/or U.S.C.G. exams.
Standards of Training, Certification and Watchkeeping (STCW) - STCW for seafarers set international standards for masters, officers and watch personnel on seagoing merchant ships. STCW training is intended to ensure that seafarers are aware of the hazards of working on a vessel and can respond appropriately in an emergency.
A STCW Certificate will be issued to mariners who meet the STCW requirements and is separate from U.S. Merchant Mariner license. STCW requires seafarers be provided with "familiarization training" and "basic safety training" which includes: basic fire fighting, elementary first aid, personal survival techniques, and personal safety and social responsibility. Mariners entering training programs after August 1, 1998, MMA license program students included, are required to meet the current international competency standards.