Maritime geography is often discussed in terms of three loosely defined regions: brown water, green water, and blue water.

Definitions

The elements of maritime geography are loosely defined and their meanings have changed throughout history. The USA's 2010 Naval Operations Concept defines blue water as "the open ocean", green water as "coastal waters, ports and harbors", and brown water as "navigable rivers and their estuaries". Robert Rubel of the US Naval War College includes bays in his definition of brown water, and in the past US military commentators have extended brown water out to 100 nautical miles (190 km) from shore.

During the Cold War, green water denoted those areas of ocean in which naval forces might encounter land-based aircraft and brown water, land-based artillery. The development of long-range bombers with antiship missiles turned most of the oceans to "green" and the term all but disappeared. After the Cold War, US amphibious taskforces were sometimes referred to as the green-water navy, in contrast to the blue-water carrier battlegroups. This distinction disappeared as increasing threats in coastal waters forced the amphibious ships further offshore, delivering assaults by helicopter and tiltrotor from over the horizon. This prompted the development of ships designed to operate in such waters - the Zumwalt class destroyer and the littoral combat ships. Rubel has proposed redefining green water as those areas of ocean which are too dangerous for high-value units, requiring offensive power to be dispersed into smaller vessels such as submarines that can use stealth and other characteristics to survive. Under his scheme brown water would be zones in which ocean-going units could not operate at all, including rivers, minefields, straits and other choke points.