Must hold Tankerman certification when working aboard liquid-carrying vessels. Those employed on Great Lakes ships work 60 days and have 30 days off, but do not work in the winter when the lakes are frozen. Workers on rivers, canals, and in harbors are more likely to have year-round work. Others work steadily for a week or month and then have an extended period off. When working, they are usually on duty for 6 or 12 hours and are off for 6 or 12 hours.
Merchant mariners spend extended periods at sea, and earn leaves.
Most are hired for one or more voyages that last for several months, although there is no job security after that voyage. Merchant marine officers and seamen, both experienced and beginners, are hired for voyages through union hiring halls or directly by shipping companies. These hiring halls prioritize the candidates by the length of time the person has been out of work, and fill open slots accordingly. Hiring halls are typically found in major sea ports. About 40 percent of all water transportation workers were employed on board merchant marine ships or U.S. Navy Military Sealift ships operating on the oceans or Great Lakes.