Which sea is the only one defined by ocean currents rather than coastlines?

The Sargasso Sea is the only sea defined by ocean currents rather than coastlines.

It lies within the North Atlantic Subtropical Gyre, bounded by the North Atlantic Current, the Gulf Stream, the Canary Current, and the North Atlantic Equatorial Current. These moving water boundaries surround a region whose surface is often marked by floating brown Sargassum seaweed.

Unlike nearly all named seas, the Sargasso Sea has no land coastline. Its position and size vary with the currents, and it is not a sharply enclosed body of water. The area is important habitat for species such as the European and American eel during parts of their life cycles, while its warm, relatively calm waters also influenced early sailing routes.