The difference in temperature, salinity and CO2 concentrations in seas cause water to separate into layers. This was only known recently.
(Quran 55.19-20) He [Allah] has made the two seas meet, between them a barrier that they don’t transgress.
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.
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%.