Upper most zone in oceans where there is visible light.
Visible light cannot penetrate deep waters. The zone which humans can see unaided is called the photic zone:
It extends from the surface down to a depth where light intensity falls to one percent of that at the surface, called the euphotic depth. Accordingly, its thickness depends on the extent of light attenuation in the water column. Typical euphotic depths vary from only a few centimetres, in highly turbid eutrophic lakes, to around 200 meters in the open ocean. It also varies with seasonal changes in turbidity.
Wikipedia, Photic Zone, 2018
So humans can only see unaided from few centimeters to 200m. However this was revealed in the Quran 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 says that it is so dark down there you cannot see your own hand.
How could an illiterate man who lived 1400 years ago have known that light cannot reach deep waters?