The Deck Department is an organizational unit aboard naval

Navy

A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...

 and merchant ships. A Deck Officer is an officer serving in the deck department.
The main work of the deck department is proper watchstanding and the maintenance of the ship's hull, cargo gear, and accommodations as well as the ship's life saving and firefighting appliances.

A typical deck department for a merchant ship would include:

• (1) Chief Officer
  Chief Mate
  A Chief Mate or Chief Officer, usually also synonymous with the First Mate or First Officer , is a licensed member and head of the deck department of a merchant ship...
  
  /Chief Mate
• (1) Second Officer
  Second Mate
  A second mate or second officer is a licensed member of the deck department of a merchant ship. The second mate is the third in command and a watchkeeping officer, customarily the ship's navigator. Other duties vary, but the second mate is often the medical officer and in charge of maintaining...
  
  /Second Mate
• (1) Third Officer
  Third Mate
  A Third Mate or Third Officer is a licensed member of the deck department of a merchant ship. The third mate is a watchstander and customarily the ship's safety officer and fourth-in-command...
  
  /Third Mate
• (0-1) Boatswain
  Boatswain
  A boatswain , bo's'n, bos'n, or bosun is an unlicensed member of the deck department of a merchant ship. The boatswain supervises the other unlicensed members of the ship's deck department, and typically is not a watchstander, except on vessels with small crews...
  
  
• (2-6) Able Seamen
  Able Seaman (occupation)
  An able seaman is an unlicensed member of the deck department of a merchant ship. An AB may work as a watchstander, a day worker, or a combination of these roles.-Watchstander:...
  
  
• (0-2) Ordinary Seamen
  Ordinary Seaman (occupation)
  An ordinary seaman is an unlicensed member of the deck department of a merchant ship. The position is an apprenticeship to become an able seaman, and has been for centuries...
  
  

Depending on the size and employment of the ship, a boatswain

Boatswain

A boatswain , bo's'n, bos'n, or bosun is an unlicensed member of the deck department of a merchant ship. The boatswain supervises the other unlicensed members of the ship's deck department, and typically is not a watchstander, except on vessels with small crews...

 may be employed.