It takes many people to crew a ship safely. At the top of all departments in the ship is, of course, the Captain. The Captain of a ship is responsible for everything that happens and every person who is on that ship. However, in the deck department, the deck crew of a ship has officers who each have a role in leadership and in keeping things running safely.
Directly under the captain in the deck crew is the First Mate. The First Mate is responsible for the crew, but still reports to the captain. The first mate also oversees the bosun, or foreman of the deck crew. The bosun is usually the most senior of the deck hands, and in this way the First Mate indirectly oversees the able seamen and ordinary seamen of the deck crew.
In addition, the First Mate has specific responsibilities relating to cargo and stability. He or she must determine where different types of cargo will be stored on the ship so that the ship is both balanced, and sitting in the water high enough to see the load line, or the line painted around a ship showing how low it can be in the water and still be safe. The First Mate mathematically calculates this, taking both the load line and the depth of different ports into account, and writes out a cargo plan.
In the deck crew, the Second Mate is the navigator. He or she has been trained in and passed a Coast Guard test in navigation, with both modern and traditional navigational equipment. The Second Mate will generally use GPS and other modern instruments to determine location and course, but can fall back upon such traditional and time-tested instruments as a sextant, like many generations of sailors before him.
The Third Mate is another officer of the deck crew, and is specifically responsible for safety. He or she maintains and stores all required firefighting and lifesaving equipment. The Third Mate is also responsible for the upkeep of all safety equipment.
In addition to their specific duties, all officers of the deck crew stand a navigational watch, which is usually a four hour shift, twice a day. The watch is generally at the same time in both a.m. and p.m. - for example, the First Mate might have the 8-12 watch in the morning and the 8-12 watch at night.