A licensed mariner is a person who holds a license issued by one or more countries to hold senior positions aboard ships, boats, and similar vessels. The United States Coast Guard grants licenses to members of the United States Merchant Marine in five categories: deck officers, In the United States the Coast Guard has replaced paper licenses with the Merchant Mariners Credential which is a combination of the former Merchant Marine Officers license and Merchant Mariners Document in a small book that looks similar to a passport.
Mariners that do not have a license are referred to as unlicensed mariners or ratings.
There are a wide variety of licenses for deck officers, with restrictions of geography and tonnage. Licenses without such restrictions are called unlimited as in Third Mate or Officer of the Watch, Unlimited. The grades of unlimited licenses are: