MarineNews contributor (Captain) Jeff Cowan handicaps the current system of medical evaluations and credentialing by the U.S. Coast Guard. It’s not all smooth sailing. “While the U.S. Coast Guard regrets any hardship merchant mariners may experience to maintain their credentials, we must maintain the safety of the Marine Transportation System through a comprehensive MMC (Merchant Marine Credential) evaluation process. This allows the U.S. The U.S. Coast Guard issued these words in response to a query made by a congressional representative on behalf of a prospective mariner regarding incorrect Medical Credential date of expiration after three weeks of no response from the National Mariner Center (NMC) in Martinsburg, WV.