I don't think Mermaids are a myth. They are water creatures just like fishes with head , upper body and the tail of a fish
I'm on the fence.  They seem like they could be real.
Mermaid
In folklore, a mermaid is an aquatic creature with the head and upper body of a female human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including the Near East, Europe, Asia, and Africa. The first stories appeared in ancient Assyria, in which the goddess Atargatis transformed herself into a mermaid out of shame for accidentally killing her human lover. Mermaids are sometimes associated with perilous events such as floods, storms, shipwrecks, and drownings. In other folk traditions
feeding on benthos and benthopelagic zooplankton. Most dermersal fish are benthopelagic.
They can be divided into flabby or robust body types. Flabby benthopelagic fishes are like bathopelagic fishes, they have a reduced body mass, and low metabolic rates, expending minimal energy as they lie and wait to ambush prey. An example of a flabby fish is the cusk-eel "Acanthonus armatus", a predator with a huge head and a body that is 90% water. This fish has the largest ears (otoliths) and the smallest