The boundary between the Pacific and Atlantic Oceans is completely unmixable due to their different densities and salinities, creating a clear, visible line in the water where the two oceans meet near Cape Horn, South America.
Discussion
This is not true, they definitely do mix and the only conflicts are temperatures, warm water from the north meeting cold water from the artic makes for extreme conditions but there's no line or color difference. The main places you'll see differences in color like this are where rivers or Gulf waters meet oceans and the sediment hasn't settled yet. The Alaskan Gulf often can look like this when the sediment fans out into the ocean.
Thanks for the correction. More accurate to say, they do not mix easily