Between the two seas is a Barrier

The Quran says:

مَرَجَ الْبَحْرَيْنِ يَلْتَقِيَانِ

بَيْنَهُمَا بَرْزَخٌ لَا يَبْغِيَانِ

فَبِأَيِّ ءَالَاءِ رَبِّكُمَا تُكَذِّبَانِ

“He has made the two bodies of water flow. They will one day meet. Between them is now a barrier; they encroach not one upon the other. Which, then, of the favours of your Lord will you twain deny?”

(Surah Ar-Rahman : 19-21)

In the Arabic text the word barzakh means a barrier or a partition. This barrier is not a physical partition. The Arabic word maraja literally means ‘they both meet and mix with each other’. Early commentators of the Qur’aan were unable to explain the two opposite meanings for the two bodies of water, i.e. they meet and mix, and at the same time, there is a barrier between them. Modern Science has discovered that in the places where two different seas meet, there is a barrier between them. This barrier divides the two seas so that each sea has its own temperature, salinity and density. Oceanologists are now in a better position to explain this verse. There is a slanted unseen water barrier between the two seas through which water from one sea passes to the other. 

(Principles of Oceanography, Davis, pp. 92-93.) 

But when the water from one sea enters the other sea, it loses its distinctive characteristic and becomes homogenized with the other water. In a way this barrier serves as a transitional homogenizing area for the two waters. This scientific phenomenon mentioned in the Quran was also confirmed by Dr. William Hay who is a well-known marine scientist and Professor of Geological Sciences at the University of Colorado, U.S.A.