How can two seas not mix?

How can two seas not mix? How can two seas not mix? I think this is commonly known and the explanation everyone gives is "because they have different densities". See more on:

And an explanation thereon:

" 7. Halocline  Salinity separation phenomena  - Halocline is a zone where there are extreme vertical changes in the salinity in the ocean. 
In lower latitudes the halocline can represent a decrease in salinity with increased depth; high altitudes it represents the opposite.

The halocline normally develops in regions where there is a great amount of freshwater input from rivers producing surface waters of low salinity, a zone where salinity increases rapidly with depth (the halocline), and a deeper zone of greater saline, dense waters. "

(Photo credit - Ocean Networks Canada)

And more images, from Google search for images:

And one may go through:

Density current

PhysicsWritten by: Claudia Cenedese

Best regards,