A-i think, that the link between seas gradually being darker when leaving shore and darkness in the depths of the seas, is only obvious because we already know of the latter.
meaning, i believe(and that's all what this is for now), that if we DID get hold of a person from that time, say a fisher man, who's always seen the sea get darker the further away we get from the shore since he was a kid, and asked him if he'd imagine that deep inside the sea, it's pitch black like a dark room, i think he'd either ponder it and agree, or straight out say you're wrong...
BECAUSE HE'S DIVED IN THE SEA, AND IT'S NOT PITCH BLACK.
but hey, it gets darker the further you go from the shore, so it's logical that at one point it'll be pitch black..
but having both black and white cats, black and white dogs, black and white pigeons, black and white goats, and so on...
...did not prevent the people of Europe from not believing in the existence of black swans, till they "eye balled" them in Australia
not only was the sea getting darker an everlasting aspect of how it(the sea) has always been to them, nothing to spark thought or skepticism... just like falling apples and newton, it's logical, but only after it's been figured out
BUT ALSO they are living a reality contrary to the fact that some places in the sea are pitch black.
i hope this is clear, although not solid proof, i think it's an explanation of why my objective mind didn't quite swallow that gradual darkening of the sea when observed from the surface by fishermen directly dictates that it was known that it's extremely dark in the unreachable depths.