46 CFR 10.203: Requirement to hold a TWIC and a Merchant Mariner Credential.

(a) Failure to obtain or hold a valid TWIC serves as a basis for the denial of an application for an original, renewal, new endorsement, duplicate, or raise of grade of a mariner's credential and may serve as a basis for suspension and revocation under 46 U.S.C. 7702 and 7703.

(b) An MMC must be retained by the mariner to whom it was issued and, while valid, must be produced to verify qualifications when requested by an authorized official as identified in 33 CFR 101.515(d). Posting of the officer endorsement may be necessary as required in 46 U.S.C. 7110.

(c) Although an MMC serves as certificates of identification, a TWIC must be retained by the mariner to whom it was issued and, while valid, serves as the mariner's primary identification document. The TWIC must be produced to verify identity when required by an authorized official as identified in 33 CFR 101.515(d).