World War II Merchant Mariner Service Act - Directs the Secretary of Homeland Security (DHS) to accept additional documentation for verifying that an individual performed honorable service as a coastwise merchant seaman during the period beginning on December 7, 1941, and ending on December 31, 1946, for purposes of eligibility for veterans' benefits under the GI Bill Improvement Act of 1977. Requires such documentation to include Social Security Administration (SSA) records and validated testimony in the case of the absence of Coast Guard shipping or discharge forms, ship logbooks, documents, or other official employment records.

Requires the Secretary, when determining whether to recognize service allegedly performed during such period, to recognize masters of seagoing vessels or other command officers who were authorized to document an individual for purposes of hiring for or discharging from the merchant marine.

Considers any service so recognized as active-duty service for purposes of veterans' burial benefits. Makes such veterans eligible for any appropriate military medals, ribbons, and decorations. Requires the Secretary to verify that an individual performed such service under honorable conditions without regard to their sex, age, or disability during the service period.