What is Coast?

Legal Definition
A coastline or a seashore is the area where land meets the sea or ocean, or a line that forms the boundary between the land and the ocean or a lake. A precise line that can be called a coastline cannot be determined due to the Coastline paradox.

The term coastal zone is a region where interaction of the sea and land processes occurs. Both the terms coast and coastal are often used to describe a geographic location or region; for example, New Zealand's West Coast, or the East and West Coasts of the United States. Edinburgh for example is a city on the coast of Scotland.

A pelagic coast refers to a coast which fronts the open ocean, as opposed to a more sheltered coast in a gulf or bay. A shore, on the other hand, can refer to parts of the land which adjoin any large body of water, including oceans (sea shore) and lakes (lake shore). Similarly, the somewhat related term "[Stream bed/bank]" refers to the land alongside or sloping down to a river (riverbank) or to a body of water smaller than a lake. "Bank" is also used in some parts of the world to refer to an artificial ridge of earth intended to retain the water of a river or pond; in other places this may be called a levee.

While many scientific experts might agree on a common definition of the term "coast", the delineation of the extents of a coast differ according to jurisdiction, with many scientific and government authorities in various countries differing for economic and social policy reasons. According to the UN atlas, 44% of people live within 150 kilometres (93 miles) of the sea.
-- Wikipedia
Legal Definition
The margin of a country bounded by the sea. This term includes the natural appendages of the territory which rise out of the water, although they are not of sufficient firmness to be inhabited or fortified. Shoals perpetually covered with water are not, however, comprehended under the name of coast. The small islands, situate at the mouth of the Mississippi, composed of earth and trees drifted down by the river, which are not of consistency enough to support the purposes of life, and are uninhabited, though resorted to for shooting birds, were held to form a part of the coast. 5 Rob. Adm. R. 385. (c).
-- Bouviers Law Dictionary
Legal Definition
The edge or margin of a country bounding on the sea. It is held that the term includes small Islands and rcefs naturally connected with the adjacent land, and rising above the surface of the water, although their composition may not be sufficiently firm and stable to admit of their being inhabited or fortified; but not shoals which are perpetually covered by the water. In S. v. Pope, 28 Fed. Cas. 630; Hamilton v. Menifee, 11 Tex. 751. This word is particularly appropriate to the edge of the sea, while "shore" may be used of the margins of inland waters.
See also
-- Black's Law Dictionary