Merchant mariners spend extended periods at sea, and earn leave. 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 veterans and beginners, are hired for voyages through union hiring halls or directly by shipping companies. 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 seaports.

At sea, these workers usually stand watch for 4 hours and are off for 8 hours, 7 days a week. 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. Some work 8- or 12-hour shifts and go home every day. 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.

People in water transportation occupations work in all weather conditions. Although merchant mariners try to avoid severe storms while at sea, working in damp and cold conditions is often inevitable. While it is uncommon nowadays for vessels to suffer sea disasters such as fire, explosion, or a sinking, workers face the possibility that they may have to abandon their craft on short notice if it collides with other vessels or runs aground. They also risk injury or death from falling overboard, and hazards associated with working with machinery, heavy loads, and dangerous cargo.

Most newer vessels are air-conditioned, soundproofed from noisy machinery, and equipped with comfortable living quarters. Nevertheless, some mariners dislike the long periods away from home and the confinement aboard ship, and consequently leave the industry.

Source: http://www.vetsuccess.gov/resources/occupations/profile?id=293