I think the saying about fish not being aware of water is nonsense. People are aware that there's air around them, without having to know any science. They can feel the wind, their own breath, etc. Fish have that plus they need to push against water in order to move. A fish that didn't know what water was would be like a land animal that didn't know what the ground was.
I think the general population always knew that there was a transparent substance all around them which had to keep going in and out of their lungs or they would die - the "breath of life" from the Bible. I mean, even an uneducated peasant in ancient times could blow bubbles underwater and see that there was something coming out of his mouth.