Visible light cannot penetrate deep waters. The zone in 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.
So humans can only see unaided from few centimeters to 200m. This was known recently, however this was portrayed in the Quran 1400 years before it was discovered:
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!
٤٠ أَوْ كَظُلُمَاتٍ فِي بَحْرٍ لُجِّيٍّ يَغْشَاهُ مَوْجٌ مِنْ فَوْقِهِ مَوْجٌ مِنْ فَوْقِهِ سَحَابٌ ۚ ظُلُمَاتٌ بَعْضُهَا فَوْقَ بَعْضٍ إِذَا أَخْرَجَ يَدَهُ لَمْ يَكَدْ يَرَاهَا ۗ وَمَنْ لَمْ يَجْعَلِ اللَّهُ لَهُ نُورًا فَمَا لَهُ مِنْ نُورٍ
The Quran says that it is so dark down there you cannot see your own hand.
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