The control and ownership of the oceans has long been a controversial topic. Since ancient empires began to sail and trade over the seas, command of coastal areas has been important to governments. However, it wasn't until the twentieth century that countries began to come together to discuss a standardization of maritime boundaries. Surprisingly, the situation still has yet to be resolved.
From ancient times through the 1950s, countries established the limits of their jurisdiction at sea on their own. While most countries established a distance of 3 nautical miles, the borders varied between 3 and 12 nm. These territorial waters are considered part of a country's jurisdiction, subject to all of the laws of the land of that country.
From the 1930s to the 1950s, the world began to realize the value of mineral and oil resources under the oceans. Individual countries began to expand their claims to the ocean for economic development.
In 1945, U.S. President Harry Truman claimed the entire continental shelf off the coast of the U.S. (which extends almost 200 nm off the Atlantic coast). In 1952, Chile, Peru, and Ecuador claimed a zone 200 nm from their shores.
The international community realized that something needed to be done to standardize these borders. The first United Nations Conference on the Law of the Sea (UNCLOS I) met in 1958 to begin discussions on these and other oceanic issues. In 1960 UNCLOS II was held and in 1973 UNCLOS III took place. Following UNCLOS III, a treaty was developed that attempted to tackle the boundary issue. It specified that all coastal countries would have a 12 nm territorial sea and a 200 nm Exclusive Economic Zone (EEZ). Each country would control the economic exploitation and environmental quality of their EEZ.
Though the treaty has yet to be ratified, most countries are adhering to its guidelines and have begun to consider themselves ruler over a 200 nm domain. Martin Glassner reports that these territorial seas and EEZs occupy approximately one-third of the world ocean, leaving just two-thirds as "high-seas" and international waters.
When two countries lie closer than 400 nm apart (200nm EEZ + 200nm EEZ), an EEZ boundary must be drawn between the countries. Countries closer than 24 nm apart draw a median line boundary between each other's territorial waters. The UNCLOS protects the right of passage and even flight through (and over) narrow waterways known as chokepoints.
Countries like France, which continues to control many small Pacific islands, now have millions of square miles in potentially profitable ocean area under their control. One controversy over the EEZs has been to determine what constitutes enough of an island to have its own EEZ. The UNCLOS definition is that an island must remain above the water line during high water and may not just be rocks, and must also be habitable for humans.
There's still much to be hammered out regarding the political geography of the oceans but it seems that countries are following the recommendations of the 1982 treaty, which should limit most arguments over control of the sea.