How does a DUI affect merchant mariner's documentation?

A: According to 46 CFR § 7703, a DUI may serve as cause to revoke, suspend or deny a Merchant Marine Credential (MMC) for three years after the conviction. If the DUI was due to alcohol abuse or addiction, the person is ineligible for an MMC unless proof of completion of an alcohol rehab program or active membership of a counseling or rehab group is submitted. The number of convictions the person has affects the different time periods of assessment. After the first DUI offense, an MMC application will not be considered for one year. The Coast Guard has discretion to decide whether or not to issue an MMC even after the assessment period depending upon the nature of the conviction. The passage of time reduces the negative effect of the event upon the application.

Source: Johnson & Johnson DUI Blog