The Merchant Mariner Credential or MMC is a credential issued by the United States Coast Guard in accordance with the STCW guidelines to United States seafarers to show evidence of a mariner's qualifications. It will become the standard documentation required for all crew members of U.S. ships with a Gross Register Tonnage of over 100 and for all vessels required to operate with a licensed Master, regardless of size. As previously issued but still valid credentials expire, the MMC will replace the Merchant Mariner's Document, merchant mariner license, Certificate of Registry, and STCW Certificate - http://en.wikipedia.org/wiki/Merchant_Mariner_Credential

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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, engineers, staff officers, radio officers, and pilots. The United States 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.
In the UK the Maritime and Coastguard Agency issue licenses, known as Certificates of Competency, in a similar fashion under the amended STCW convention.- http://en.wikipedia.org/wiki/Licensed_mariner