Could be real if say, a traditional native culture put effigies of fish on their heads for some reason, and if it was on an attractive naked native girl, and foreign sailors saw it, they may remember it well, and embellish the story.
Religion: Oannes came from the sea to teach humanity, he apparently was part fish and part man. I think it is related to why the pope wears a fish on his head.
In the myths about Mu, Lamaria, and Atlantis, some weird stuff is claimed. If it is all from another time or dimension, maybe some of it is true; and sailors saw things through a time warp or by other unusual means.
What would ancient and future people say about the bed ridden super obese people of our age, needing a crane to leave their abode; half whale half human ?
Or the body art freaks with tattoos and skin inserts, looking anything but standard human.