WhereHas a real mermaid ever been found?

No. Mermaids are mythological creatures. However, there have been many reported sightings of them, and hoaxes involving their existence.

Sirenia is an order of fully aquatic, herbivorous mammals that inhabit rivers, estuaries, coastal marine waters, swamps, and marine wetlands. Animals belonging to this order include manatees and the dugong. Before mid 19th century, mariners referred to these animals as mermaids.

Obviously, it is impossible to entirely dispell the possibility of there being mermaids in the sea! But the general concensus is that there are not.