I’m going to assume that is just a woman in a mermaid suit but the way this would work is that they would normally be hidden from us like little people are and various other mythological creatures. Probably in hidden dimensions and not just invisible which is also possible for people to be. They could look like anything due to illusions too and maybe not really as pretty as they appear.
Also if I saw a mermaid I would swim to shore like my life depended on it because they’re said to be cannibals so probably scary looking. If you don’t believe that, maybe you could consider these are just from aliens playing tricks on us.
Have you heard the ufo/alien stories about Antarctica? There is one in the big lake there where people said they came across alien type creatures underwater and they used some type of technology to make them rise to the surface and some got Benz disease because they were made to come up too quickly.
Or you can bring up satyrs of Ancient Greece that are goat people. Jesus says he will divide people into sheep and goats. I have seen out of corner of my eye a girl that had goat legs. When I looked at her directly she appeared normal.