I have been researching the life of a professional seafarer to see if this is indeed the career route I now wish to follow. Since others might also find this information useful, I'm putting a summary of what I compile online. I'll develop this page further as I learn more about the wide and various positions available to the professional mariner.
Captain (Master, Skipper): The person in overall command of the operation of a vessel. They supervise the work of the other officers and the crew. They set course and speed, maneuver the vessel to avoid hazards and other ships, and periodically determine its position using navigation aids, celestial observations, and charts. They direct crew members who steer the vessel, operate engines, signal to other vessels, perform maintenance and handle lines, or operate towing or dredging gear. Captains insure that proper procedures and safety practices are followed, check that machinery and equipment are in good working order, and oversee the loading and unloading of cargo or passengers. They also maintain logs and other records of ships' movements and cargo carried.
Deck Officers (Bridge Officers, Mates): Assistants to the captain. Merchant marine vessels - those carrying cargo overseas - have a chief or first mate, a second mate, and a third mate. Mates oversee the operation of the vessel, or "stand watch" for specified periods, usually 4 hours on and 8 off. On smaller vessels, there may be only one mate (called a pilot on some inland vessels) who alternates watches with the captain.
Deckhands (Seamen): Operate the vessel and its deck equipment under the direction of the ship's officers, and keep the non-engineering areas in good condition. They stand watch, looking out for other vessels, obstructions in the ship's path, and aids to navigation. They also steer the ship, measure water depth in shallow water, and maintain and operate deck equipment such as life boats, anchors, and cargo-handling gear. When docking or departing, they handle lines. They also perform maintenance chores such as repairing lines, chipping rust, and painting and cleaning decks and other areas. Seamen may also load and unload cargo. On vessels handling liquid cargo, they hook up hoses, operate pumps, and clean tanks. Deckhands on tugboats or tow vessels tie barges together into tow units, inspect them periodically, and break them apart when the destination is reached. Larger vessels have a boatswain (bosun) or head seaman.
Purser: The main purpose of the purser is to see to the comfort of the passengers. This position would only be found on cruise ships, ferries or similar vessels transporting paying passengers.
Engineers: They operate, maintain, and repair propulsion engines, boilers, generators, pumps, and other machinery. Merchant marine vessels usually have four engineering officers: A chief engineer and a first, second, and third assistant engineer. Assistant engineers stand periodic watches, overseeing the operation of engines and machinery.
Marine Oilers: People who work below decks under the direction of the ship's engineers. They lubricate gears, shafts, bearings, and other moving parts of engines and motors, read pressure and temperature gauges and record data, and may repair and adjust machinery.
Pilots: They guide ships in and out of harbors, through straits, and on rivers and other confined waterways where a familiarity with local water depths, winds, tides, currents, and hazards such as reefs and shoals is of prime importance. Pilots on river and canal vessels usually are regular crew members, like mates. Harbor pilots are generally independent contractors, who accompany vessels while they enter or leave port. They may pilot many ships in a single day.
A typical deep sea merchant ship has a captain, three deck officers or mates, a chief engineer and three assistant engineers, plus six or more seamen and oilers. Depending on their size, vessels operating in harbors, rivers, or along the coast may have a crew comprising only of a captain and one deckhand, or as many as a captain, a mate or pilot, an engineer, and seven or eight seamen. Large vessels also have a full-time cook and helper, while on small ones, a seaman does the cooking. Merchant mariners also have an electrician, machinery mechanics, and a radio officer.
Merchant mariners are away from home for extended periods, but earn long leaves. Many are hired for one voyage, with no job security after that. At sea, they 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 over. Workers on rivers and 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.
Some of the information I've read says that jobs in the marine industry are low paying and hard to come by. This is mostly coming from US sources. Canadian reports say there is a shortage of over 40,000 people worldwide, and that you can expect above average income.
NOTE: Much of the information contained herein was obtained from Water Transportation Occupations. This was the best source I found on the web to date.