Answer:
Unless there is some specific term referring to visible differences in color, turbidity, or transparency, I would use the following terms:
“Straits”, the narrow waterway past the tip of a continent, dividing one ocean or sea from another.
“The Equator”, dividing North Atlantic from South Atlantic, and dividing North Pacific from South Pacific.
Plus a very recent definition: “a 9 dashed line” around the South China Sea.
