The only sea on Earth with no coastline is bounded entirely by ocean currents

Every other sea is defined at least partly by land, but the Sargasso Sea in the North Atlantic is ringed only by currents: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south. These currents trap floating Sargassum seaweed inside a calm gyre — and it's where European and American eels travel thousands of kilometres to spawn.

— NOAA National Ocean Service, What is the Sargasso Sea? — NOAA Ocean Facts