All mariners that are required to obtain Coast Guard credentials are required to obtain a TWIC from the U.S. Department of Homeland Security. This credential states that you are a U.S. citizen or a permanent resident and have passed a security screening.
In addition, with few exceptions, the Coast Guard requires that mariners applying for a credential after April 15, 2009, obtain a MMC. Entry level seamen or deckhands on vessels operating in harbors or on rivers or other waterways do not need a MMC. The MMC replaces the Merchant Mariner Document, the license and Standards of Training, Certification and Watchkeeping for Seafarers endorsement. The MMC incorporates the licenses into the credential, which varies by occupational specialty, type of vessel and by body of water (river, inland waterway, Great Lakes and oceans). Requirements for the credential increase as the skill level of the occupational specialty and the size of the vessel increase and applicants must pass a test in order qualify. Applicants for the credential must also pass a drug screen, take a medical exam and meet the minimum age requirements. For more information on credentialing requirements see the Coast Guard's website listed in the sources of additional information.
Radio operators are licensed by the Federal Communications Commission.