Why do the two oceans not mix? The oceans never mix..
An unseen curtain separates the bodies of water whose salt and heat level are different from each other in places where the seas meet each other or where the rivers flow into the seas. This fact, which was discovered in this century, was expressed centuries ago in the 19th and 20th verses of Chapter al-Rahman;
“He has given freedom to the two great bodies of water, so that they might meet: [yet] between them is a barrier which they may not transgress.” (al-Rahmân, 55: 19-20; See also al-Furqân, 25: 53)
Source: The Universe, The Qur’an And The Human, Osman Nuri Topbaş, Erkam Publications