This was intended for zizinho.
At the time of Aristotle, people knew that waters of different density and salinity do not mix. A thousand years before Muhammad Aristotle wrote the following: "The drinkable, sweet water, then, is light and is all of it drawn up: the salt water is heavy and remains behind." Obviously the sailors navigating in that sea must have noticed the difference of color between the waters.
The verse indicates that "He" didn't understand the phenomenon at all. The sill of Gibraltar, the barrier that separates the Atlantic from Mediterranean is not responsible for not letting the two waters mix. The waters do not mix because of the difference in their density, salinity and temperature. Besides the separation of these waters are not permanent. The waters of various densities, salinity's and temperatures eventually mix. The phenomenon is short lived and is observable only when the two bodies of water meet.
Muhammad is not talking about the water of the Atlantic and the Mediterranean Sea. There is no mention of them in the Qur'an, so, it could be any two bodies of water. Was this phenomenon unknown to people at the time of Muhammad? No! This is an observable phenomenon. Anyone, who has seen a delta where rivers carrying colored sediments enter the sea or where two rivers of different colors or two seas meet, can see that for a long distance the two waters keep apart. There is no reason to believe that Muhammad had not heard of this wonder from those who had observed it. All seamen who shared their stories of their adventures had this knowledge.
The verse is not a miracle. It is a statement of something that was known being impressed but without understanding the physics behind it. It is clear to have been thought to be the work of a deity called Allah that keeps the waters apart.