Mermaids are based on marine mammals, especially manatees and dugongs. Because of their slender bodies and small heads, and the refraction of water, people look into the water from land, just like species with fish tails swimming in the water. So there is the legend of mermaid.
But it takes a hammer to prove the existence of a species. For example, we found dinosaur fossils, which proved the fact that this great man once ruled the earth.
So if you want to prove the existence of mermaids, unless you find fossils or someone actually photographed them. But these two aspects have not been confirmed.
Therefore, I don't think mermaids exist.