The difference in temperature, salinity and CO2 concentrations in seas cause water to separate into layers. This was only known recently. However Moslems knew about it all along:
(Quran 55.19-20) He [Allah] has made the two seas meet, between them a barrier that they don't transgress.
How could an illiterate man who lived 1400 years ago could have known that sea water can separate?
Not only light cannot reach deep waters but also the water layers down there are not flat but wavy, that is, the seas have internal waves just as they have surface waves. However Moslems also knew about this long before it was discovered:
(Quran 24.40) Or like the depths of darkness in a vast deep ocean, overwhelmed with waves topped by waves, topped by clouds: depths of darkness, one above another: if a man stretches out his hand, he will not see it! If Allah does not give light to a person he will not have light!
The Quran talked about internal waves long before it was discovered. Also, sunlight cannot penetrate more than few hundred meters into the seas. It is so dark down there that you cannot see your own hand. Moslems ask how could an illiterate man who lived 1400 years ago have known that seas separate, have internal waves and there is no light down there?
The water cover about 71% of the Earth's surface. This is also the same ratio the word "water" and word "Land" appear in the Quran. "Water" 32 times and "Land" 13 times. The ratio of "Water" to the total (Water + Land) = 32/(32+13) = 71%.
(The Christian Bible says that Earth floats over water Psalms 136:6)