I think it probably has to do more with the amount of sunlight that filters down through the water. I assume that most fish that live anywhere near the surface need sunlight the same way we do. And if its about warm/cold water, the further down you go, the colder it gets.
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Why is it that if someone tells you that there are 1 billion stars in the universe you will believe them,
but if they tell you a wall has wet paint you will have to touch it to be sure?