They operate and maintain deep sea merchant ships, tugboats, towboats, ferries, dredges, excursion vessels, and other waterborne craft on the oceans, the Great Lakes, in harbors, on rivers and canals, and on other waterways. Captains or masters are in overall command of the operation of a vessel and they supervise the work of any other officers and crew. They determine the course and speed, maneuver to avoid hazards, and continuously monitor the vessel's position using charts and navigational aides. They either direct or oversee crew members who steer the vessel, determine its location, operate engines, communicate to other vessels, perform maintenance, handle lines, or operate vessel equipment. Deck officers or mates perform the work for captains on vessels when they are on duty.
Mates also supervise and coordinate activities for the crew aboard the ship.
They inspect the cargo holds during loading to ensure the load is stowed according to specifications. Mates supervise crew members engaged in maintenance and the primary up-keep of the vessel. All mates stand watch for specified periods, usually 4 hours on and 8 hours off. The mate would assume command of the ship if the captain became incapacitated. When more than one mate is necessary aboard a ship, they are typically designated Chief Mate or First Mate, Second Mate, and Third Mate.