Which sea has no coastline at all, its boundaries drawn by ocean currents rather than by land?

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The Sargasso Sea floats inside the North Atlantic gyre, penned in on all sides by currents including the Gulf Stream, and takes its name from the mats of Sargassum seaweed drifting on its unusually clear blue water. Both European and American eels swim thousands of kilometres across the Atlantic to spawn there and nowhere else.