"Salt water covers more than two-thirds of the earth's surface. These waters comprise an immense maritime domain, a continuous body of water that is the earth's greatest defining geographic feature. Ships that ply the maritime domain are the primary mode of transportation for world trade, carrying more than 80 percent of the world's trade by volume. United States maritime trade is integral to the global economy, representing more than 20 percent3 of global maritime trade. Through the Maritime Transportation System (MTS), the maritime mode is the primary transportation mode providing connectivity between the U.S. and global economies; 99 percent of overseas trade by volume enters or leaves the U.S. by ship. The MTS enables the U.S. to project military presence across the globe, creates jobs that support local economies, and provides a source of recreation for all Americans. The Nation's economic and military security are fundamentally linked to the health and functionality of the MTS. The security of the MTS is paramount to protecting the Nation and its economy, but it presents daunting and unique challenges to managers of the Maritime Mode. Security of the MTS is intrinsically linked to the security of the maritime domain which contains critical infrastructure and key resources (CI/KR) from many of the other critical infrastructure sectors and Transportation Sector modes. Providing for the security of the MTS depends upon understanding the diverse array of activities in the maritime domain through the transparency of all sector and transportation modal infrastructure and security activities."
Dept. of Homeland Security: http://www.dhs.gov/