How can tell where one ocean ends and another one begins

Oceanographer : We make the distinction between different ‘oceans’ out of convenience — the commonly used boundaries are arbitrary and are set by the International Hydrographic Organisation based largely on geography and history rather than any oceanographic basis.

It is completely false to claim that there is a clear physical or visible boundary between the different ‘oceans’. Although different ‘oceans’ have different typical physical properties, there is no sharp boundary, and it’s certainly not visible. When people see a sharp change in the appearance of the ocean, they’re looking at a small-scale transient feature like a front or sediment plume.

The answer also isn’t currents. Although most of the major surface currents are confined within different ‘oceans’, this is largely because both currents and nomenclature are strongly influenced by bathymetry, rather than nomenclature being set on the basis of currents. Currents are not static, do not respect our ‘boundaries’, and in some cases cut directly across them (e.g. the Indonesian Throughflow).