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

Brown water

The brown water environment consists of the littoral

Littoral

The littoral zone is that part of a sea, lake or river that is close to the shore. In coastal environments the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas that are permanently submerged. It always includes this intertidal zone and is often used to...

 areas, from the coast and estuarial

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 areas to perhaps a hundred miles from shore. It is the most important maritime arena, including all coastal traffic and territorial waters, in which are found the great majority of a nation’s maritime police

Water police

Water police, also called harbour patrols, port police, marine/maritime police, nautical patrols, bay constables or river police, are police officers, usually a department of a larger police organisation, who patrol in water craft...

, customs, environmental, and economic concerns.

A "brown-water navy

Brown-water navy

Brown-water navy is a term that originated in the United States Navy, referring to the small gunboats and patrol boats used in rivers, along with some of the larger ships that supported them as "mother ships," from which they operated...

" focuses on coastal operations and primarily takes a defensive role.

Green water

The green water environment extends from the outer edge of the brown-water layer past any continental shelves

Continental shelf

The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

, archipelago

Archipelago

An archipelago , sometimes called an island group, is a chain or cluster of islands. The word archipelago is derived from the Greek ἄρχι- – arkhi- and πέλαγος – pélagos through the Italian arcipelago...

s and island

Island

An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...

s; perhaps a thousand miles from shore.

A "green-water navy

Green-water navy

A Green-Water Navy is a naval term that refers to a naval force whose power projection capability is regional in nature. This is a relatively new terminology, as earlier non blue-water navies used to be collectively referred to as brown-water navy....

" is capable of defense of its nation in depth and is a significant offensive force within its territory.

Blue water

The blue water environment extends from the outer edge of the green-water zone through the global deep ocean

Ocean

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

.

A "blue-water navy

Blue-water navy

The term blue-water navy is a colloquialism used to describe a maritime force capable of operating across the deep waters of open oceans. While what actually constitutes such a force remains undefined, there is a requirement for the ability to exercise sea control at wide ranges...

" can project its nation's power throughout the world. The blue water policy was a long-standing political philosophy in Britain in the eighteenth century, which sought to advance British power through use of the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

. Today the United States has by far the largest blue water navy, with 11 nuclear aircraft carrier task groups.