The Sargasso Sea has no coastline at all, only a ring of currents
Most seas are hemmed in by land, but the Sargasso Sea is bounded by nothing except the circling North Atlantic Gyre. Meanwhile the Sea of Galilee is actually a freshwater lake. There is no firm line between a sea and an ocean, and international law treats the whole ocean as sea.
Sea carries two meanings: the single connected body of salt water spanning most of the planet, and particular named waters, such as marginal seas like the Mediterranean or large, nearly enclosed basins. Seas are usually smaller than oceans and saltier and larger than lakes, but the boundaries are fuzzy, and the main UN treaty on maritime law counts all ocean water as sea.
Earth is the only planet known to have liquid seas on its surface, though Mars has ice caps and distant worlds may have oceans. About 1,335,000,000 cubic kilometres of seawater hold roughly 97.2 percent of Earth's water and cover some 71 percent of its surface. Ice holds about 2.15 percent, and the remaining 0.65 percent or so is the groundwater, vapour, clouds, rain, lakes and rivers that land life depends on.
Salinity is lower at the surface and near big river mouths and higher in the depths, but the proportions of dissolved salts barely change from ocean to ocean. Sodium chloride dominates, with magnesium, calcium and potassium salts and traces of other elements. Life fills every zone, from sunlit reefs to the cold, crushing abyss and the water beneath polar ice, and many major groups of organisms evolved in the sea, where life itself may have begun.
The ocean steadies the climate and drives the water, carbon and nitrogen cycles. Winds push surface currents, while the deep global conveyor belt carries polar water into every basin. Tides, usually twice daily, come from Earth's spin and the pull of the Moon and, less so, the Sun. Undersea earthquakes, volcanoes, landslides and meteorite strikes can unleash tsunamis. People have fished, farmed, traded, fought and played on the sea since prehistory, studying it as oceanography, and many of those activities pollute it.
Source: Sea
