About the Crewman Visa - C1, D category

Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. Crewmember (D) visas are nonimmigrant visas for persons working on board sea vessels or international airlines in the United States, providing services required for normal operation and intending to depart the United States on the same vessel or any other vessel within 29 days. If you travel to the United States to join the vessel you will work on, in addition to a crewmember (D) visa, you also need a transit (C-1) visa or a combination C-1/D visa.

Learn More Here: http://travel.state.gov/content/visas/english/other/crewmember.html