What is the only sea that has no coastline?

The Sargasso Sea is the only sea that has no coastline.

The Sargasso Sea is a region of the North Atlantic defined by ocean currents rather than land boundaries. It lies within the North Atlantic Subtropical Gyre, broadly surrounded by the Gulf Stream, North Atlantic Current, Canary Current, and North Atlantic Equatorial Current. Its name comes from Sargassum, a floating brown seaweed that can form extensive mats at the surface.

Unlike conventional seas, the Sargasso Sea has no fixed shore. Its boundaries shift with the circulation of the surrounding water, so maps may show slightly different outlines. The region provides important habitat for juvenile sea turtles, eels, seabirds, and other marine life. Portuguese sailors noticed floating seaweed there during the Age of Discovery, and Christopher Columbus also recorded Sargassum in his 1492 voyage journals. The Sargasso Sea is not the same as a named coastal sea such as the Coral Sea or Arabian Sea.