Аль-Фуркан 25:53

وَهُوَ الَّذِي مَرَ‌جَ الْبَحْرَ‌يْنِ هَٰذَا عَذْبٌ فُرَ‌اتٌ وَهَٰذَا مِلْحٌ أُجَاجٌ وَجَعَلَ بَيْنَهُمَا بَرْ‌زَخًا وَحِجْرً‌ا مَّحْجُورً‌ا ﴿53﴾

And He is the One who joined the two seas - this is sweat, very sweat and this is bitter, very bitter - and made between them a buffer and a barrier, prohibited (to cross) - 25:53.

The word مَرَج (maraja) means to let off or allow to roam freely and hence pasture is called رَج (maraj) that is where animals can graze and roam about. عَذْبٌ ('adhb) is sweet water, while فُراَت means delicious and pleasant; and مِلح (milh) means salty and أُجَاجٌ means bitter.

Allah Ta’ ala in His own Wisdom has created two types of waters. One is enormous in size called oceans and cover up about two third of the global space while the rest of the one third space is made up of land mass on which people live. The water of the oceans and seas is extremely brackish, bitter and of a bad taste. On the other hand the water found on land in different forms is potable, pleasant and tasty. This water is just right for the needs of mankind and animals alike who drink it and use it for cleaning. If the water of the seas and oceans were not brackish, it would have decayed very quickly leaving behind a pungent and foul odor making the life unbearable for humans and animals on land. Moreover, the creatures living in ocean and sea water, and number manifold than those living on land could not have survived because when they die, they perish in the brackish water. If they were to die in sweet water their decomposition would have contaminated the water making the survival of the marine life impossible and its stink would have been unbearable for the life on land. Then the garbage of the land is generally dumped in the sea where it is decomposed (except for a few modern age items which need special treatment). Therefore, brackish water of the oceans and seas is a great blessing of Allah Ta’ ala for all creatures, whether living in the sea or on the land. Without the presence of brackish water mass, there would have been no marine or land life.

In this verse it is elucidated that it is a great blessing and grace of Allah Ta` a1a to have provided two types of waters in accordance with the needs of mankind. Then it is a perfect demonstration of His complete power that when a river of sweet water falls into the sea, the two waters .(the sweet and the brackish) do not mingle immediately where they meet, but for miles they remain separated from each other, despite the fact that there is no physical curtain between them. This is yet another example of His omnipotence.