I’ve seen images like this resurfacing recently claiming to be the boundary between two oceans - this is absolutely not the case. This photo comes from the Gulf of Alaska where fresher, sediment rich coastal water (light blue) and saltier, denser offshore water (dark blue) meet.
Despite the worlds oceans having different properties (on average) their waters do not just sit separately, no matter how much people throw around words ‘halocline’ or ‘thermocline’. They are subject to all sorts of turbulent mixing processes from the wind and tides etc., and even in the absence of outside forces you still get mixing via diffusion and convection (see Double Diffusive Convection and Salt Fingering).
The oceans are a wonderfully dynamic and complex system, much more so than people spreading lies and disinformation as ‘facts’ bother to even care about.
Photograph by Ken Bruland