With over 50,000 ocean going vessels out there, that could make it to North America, the U.S. Navy and Coast Guard have developed techniques to deal with the situation. First, only a few thousand ocean going ships regularly visit North America. Most are huge cargo vessels or tankers, constantly moving back and forth between East Asian or Persian Gulf ports and North American destinations. These are easy to watch. So the navy has established a class of ships that get special attention. These are called vessels of interest (VOI). Some get included in this list because they carry hazardous materials (explosives or very dangerous chemicals). Others are VOIs because they are where they shouldnt be there, or are simply suspected of involvement in one seagoing criminal activity or another. If a Chinese or African coastal freighter is spotted approaching North America, it becomes a VOI. If a sailor jumps ship in the United States, that vessel becomes a VOI (because this is now considered a method for smuggling terrorists into the country.) More attention is paid to theft at containers ports. Its long been common for criminals to smuggle goods (usually drugs) and people (often prostitutes, or just illegal migrants) in via shipping containers. But terrorists could also come in that way. Appeals to port workers patriotism usually provides a steady supply of tips on which crooks are, or might be, crossing the line from thieving to terrorism.
The search for VOIs has also uncovered a lot more nefarious activity on the high seas than the navy had previously suspected. While it was known that North Korea had been shipping illegal goods (drugs, counterfeit cash, weapons) around on its merchant ships, the VOI search uncovered much, much more. The North Koreans have been more active in gunrunning and smuggling illegal raw materials (ore, oil and lumber) out of Africa and Asian hot spots. So far, theres been no proof that the North Korean smuggling fleet has been servicing terrorist organization. But its thought the North Koreans would, if the price was right, and the chances of getting caught seemed minimal.
VOIs have become a seagoing version of the usual suspects. The same ships keep showing up again and again when the navy, coast guard or port authorities go looking for bad behavior.