Maritime geography is often discussed in terms of four loosely-defined regions: Riverine, brown water, green water, and blue water.
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The riverine environment consists of all navigable rivers of interest.
The brown water environment consists of the littoral areas, from the coast and estuarial 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, customs, environmental, and economic concerns.
A "brown-water navy" focuses on coastal operations and primarily takes a defensive role.
The green water environment extends from the outer edge of the brown-water layer past any continental shelves, archipelagos and islands; perhaps a thousand miles from shore.
A "green-water navy" is capable of defense of its nation in depth and is a significant offensive force within its territory.
The blue water environment extends from the outer edge of the green-water zone through the global deep ocean.
A "blue-water navy" 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. Today the United States has by far the largest blue water navy, with 11 nuclear aircraft carrier task groups.