And He it is Who has let free the two seas, one that quenches thirst, sweet; and the other saltish, bitter. And He has made between them a barrier and prevented (them from getting mixed up, by a) prevention {2}.
Refer to Note No.2. ‘Explanatory Notes’.
Barrier Between Two Seas:
This phenomenon is well known and often seen whereby immediate mixing of salty sea water and fresh river water does not occur. The Quran refers to this in the case of what is thought to be the estuary of the Tigris and Euphrates where they unite to form what one might call a sea over 100 miles long, the Shatt Al Arab. Rivers with very large outflow, such as Mississippi and the Yangtze, have the same peculiarity: the mixing of their fresh water with the salty water of the sea does not often occur until very far out at sea. (The Bible, The Quran and Science Maurice Bucaille. American Trust Publication 1977, PP 179-180). Differences in surface tension might be the reason for non-mixing of the two waters.