Surah Al-Furqan 25:53

۞ وَهُوَ ٱلَّذِى مَرَجَ ٱلْبَحْرَيْنِ هَـٰذَا عَذْبٌ فُرَاتٌ وَهَـٰذَا مِلْحٌ أُجَاجٌ وَجَعَلَ بَيْنَهُمَا بَرْزَخًا وَحِجْرًا مَّحْجُورًا

And it is He who has released [simultaneously] the two seas [i.e., bodies of water], one fresh and sweet and one salty and bitter, and He placed between them a barrier and prohibiting partition.

Translation: Saheeh International

Tafsir

And He it is Who merged the two seas letting them flow one adjacent to the other this one palatable sweet and the other saltish bitter; and He set between the two an isthmus so that the one does not mix with the other and a forbidding ban a shield that prevents the two from becoming mixed.

Excerpt from Tafsir al-Jalalayn, commentary on 25:53, via Quran.com. The reader has the full commentary and 9 more tafsirs.