Assalamu Alaikkum W.W.
It is already stated in the Holy Quran (Surah Arrahman, chapter 55) that there is a barrier in between 2 seas where it meets, and each one got it’s own quality (taste, specific gravity, salt content etc). What ever the reason the scientists can give now, but it has been revealed 1500 years ago by almighty Allah.

CH.55 AR-RAHMAN

20.”HE HAS MADE THE TWO BODIES OF WATER FLOW. THEY WILL ONE DAY MEET;
21. BETWEEN THEM THERE IS AT PRESENT A BARRIER; THEY CANNOT ENCROACH ONE UPON THE OTHER.
See the pictures