Mermaids appear in British folklore as unlucky omens—both foretelling disaster and provoking it.

In China a 15th-century compilation of quotations tells of a mermaid who "wept tears which became pearls".

The logbook of Blackbeard, an English pirate, records that he instructed his crew on several voyages to steer away from charted waters which he called "enchanted" for fear of merfolk or mermaids, which Blackbeard and members of his crew reported seeing.

In July 2012, the National Ocean Service (a branch of National Oceanic and Atmospheric Administration) stated that "no evidence of aquatic humanoids has ever been found".[54] The statement was a response to public inquiries following Mermaids: The Body Found, a docufiction television film which aired in May on Animal Planet and which some had mistaken for a factual documentary.

Mermaids are sometimes depicted as perilous creatures associated with floods, storms, shipwrecks, and drowning. In other folk traditions (or sometimes within the same tradition) they can be benevolent, bestowing boons or falling in love with humans.