Not outside of books as far as I know. I think that the current idea is that the mermaid myth came about because people saw manatees and dugongs, although how anyone would think a manatee or a dugong has a female torso I have no idea.
I don’t think so. In the olden days, sailors would spend weeks at sea and get very lonely as there were no women on the ships. So maybe they hallucinated or fantasised that they saw beautiful mermaids.