Merchant Mariner Tax?

No. Income earned anywhere in the world is not exempt from U.S. tax merely because of being earned somewhere other than the U.S. Even if you were off the ship and working in a factory in the foreign country, your earnings there would be subject to U.S. tax. Even if you were off the planet and working on a spacecraft, your earnings there would be subject to U.S. tax. The only places where you can avoid any federal tax because of your location are (1) if you are in a combat zone and in the military, or (2) if a treaty between the foreign government and the U.S. government says that you must pay only foreign tax and not U.S. tax (in that situation, you would still have to pay tax on the pay, just not to the U.S. government). To clarify, the combat zone pay exclusion is the only way that you would not have to pay any country's tax on the money, and that exclusion applies only to the U.S. military.

as a US citizen you are taxed on world wide income and as a merchant mariner, you do have some deductions you can use to reduce your taxable income