Maritime geography categorizes marine environments into three regions: brown water, green water, and blue water. These distinctions are fluid and have evolved over time.
Brown water encompasses navigable rivers, estuaries, and bays, extending from the shoreline to the outer edge of the continental shelf. Brown-water navies prioritize littoral defense and operate in shallow waters. They play a role in protecting waterways, ports, and coastal areas.
Green water spans coastal waters beyond the brown-water zone, including archipelagos and islands. It is a crucial maritime arena, as it encompasses territorial waters, maritime traffic, and environmental concerns. Green-water navies can defend their nation's coastline, project power, and conduct offensive operations within their territory.
Blue water refers to the vast expanse of the deep ocean beyond the green-water zone. Blue-water navies have the capability to operate and project power globally. During the Cold War, the distinction between green and blue water was blurred due to the threat of land-based aircraft and artillery. However, post-Cold War advances in stealth and long-range missiles have led to a resurgence of green-water warfare.