(This statement above is not true). There are other international laws and treaties that the US and other nations may invoke to inspect the vessel, its crew, and cargo. Under the most current round of UN sanctions against North Korea that the article refers to, the resolution does not expressly authorize opposed or non compliant boardings. UN sanctions are only one avenue, I suggest reading up on initiatives such as the Proliferation Security Initiative (PSI) at http://www.state.gov/t/isn/c10390.htm. Whether or not a country decides to actually use the authority is another question, as exemplified by the statements of Admiral Mullen today.