"He has let free the two seas meeting together. There is a barrier between them., they do not transgress. Then which of the Blessings of your Lord will you both (jinns and men) deny?" (Al-Qur'an - Surah Ar-Rahman : 19-21)
But when the Qur'an speaks about the divider between fresh and salt water, it mentions the existence of 'a forbidding partition' with the barrier.
Allah says in the Qur'an:
"He is the one who has let free the two bodies of flowing water, one sweet and palatable, and the other salty and bitter. And He has made between them a barrier and a forbidding partition." (Al-Qur'an - Surah Al-Furqan : 53)
And who wrote about it? A desert man that did not know nothing about oceanography 1400+ years ago , Muhammad pbuh.