Marine airfare eligibility requires you to be an employee traveling to/from, or in conjunction with a maritime vessel, including offshore rigs and installations that can move under their own power.
Generally this will be a seaman or merchant marine carrying seaman papers; but there are cases where eligible employees will not have seamans papers. Employees then must carry documentation of eligibility in a form of a current seamans book or a written declaration letter. A sample letter may be obtained from an Avia International travel counselor.