Appears 3 times in the Quran.
25:53 وَهُوَ ٱلَّذِى مَرَجَ ٱلْبَحْرَيْنِ هَٰذَا عَذْبٌ فُرَاتٌ وَهَٰذَا مِلْحٌ أُجَاجٌ وَجَعَلَ بَيْنَهُمَا بَرْزَخًا وَحِجْرًا مَّحْجُورًا
And He it is Who has made two seas to flow freely, the one sweet that subdues thirst by its sweetness, and the other salt that burns by its saltness; and between the two He has made a barrier and inviolable obstruction.
35:12 وَمَا يَسْتَوِى ٱلْبَحْرَانِ هَٰذَا عَذْبٌ فُرَاتٌ سَآئِغٌ شَرَابُهُۥ وَهَٰذَا مِلْحٌ أُجَاجٌ وَمِن كُلٍّ تَأْكُلُونَ لَحْمًا طَرِيًّا وَتَسْتَخْرِجُونَ حِلْيَةً تَلْبَسُونَهَا وَتَرَى ٱلْفُلْكَ فِيهِ مَوَاخِرَ لِتَبْتَغُوا۟ مِن فَضْلِهِۦ وَلَعَلَّكُمْ تَشْكُرُونَ
And the two seas are not alike: the one sweet, that subdues thirst by its excessive sweetness, pleasant to drink; and the other salt, that burns by its saltness; yet from each of them you eat fresh flesh and bring forth ornaments which you wear; and you see the ships cleave through it that you may seek of His bounty and that you may be grateful.
If We pleased, We would have made it salty; why do you not then give thanks?