Most people don't think of mermaids as monsters, per ce. In fact, mermaids have morphed in popular culture from uncommunicative, blond haired harbingers of storms at sea to red haired moppets who want only to walk and talk just like humans. (Who remembers that The Little Mermaid died in Hans Christian Anderson's story? Me too!)
Seafaring lore is full of stories of merfolk, though. Up until the Enlightenment, merpeople were taken as fact and there were actually two classifications in Ancient and Medieval science: the one with the human torso and the tail like a dolphin and another that looked entirely human but was seen too far away from shore to be a person out for a swim. Both were considered relatively benign except if hooked or caught in a net. Then there was their singing. The mermaid's song called up storms, and ships would hurry to safe harbor if one of their company claimed to hear one or more of the female merpeople singing.
Merfolk, as reported by sailors in particular, seem to be curious about humans although there is never any mention of spoken communication between the species. They would swim to boats and stare up at the men on board as if they couldn't quite figure out what the hell had invaded their territory. An account from Henry Hudson's log kept during his second search for the Northwest Passage is almost ubiquitous:
This morning, one of our company looking overboard saw a Mermaid, and calling up some of the company to see her, one more came up, and by that time she was come close to the ship's side, looking earnestly on the men.
The mermaid, claimed to have been seen clearly by able seaman Robert Raynar and Thomas Hilles in the forenoon watch, was described as the usual: human in appearance "from the navel upward" with a tail "like a porpoise". It should be noted that there is no reason to believe either or both sailors were drunk, the first grog ration not being served out until dinner, which would be had after noon. As John Michael Greer says in his 2001 book Monsters: An Investigator's Guide to Magical Beings:
If Hilles and Rayner had reported any other kind of marine life, it's worth noting, their report would be accepted as valid evidence by modern scholars without a second thought.
Compellingly put. But what about the old story of seals, manatees or dugongs being mistaken for mermaids? I suppose that a lubber by land might mistake a curious seal or manatee for a woman with a tail... I suppose. Experienced sailors though, unless they were stupid hallucinating drunk, would certainly know the difference. Then, too, there is the reality that the documentation of mermaid sightings comes overwhelmingly from the North Atlantic. Only seals and whales populate those cold waters.
Am I saying merfolk are real? Not at all, but it's something to ponder. The ocean still has her secrets. And that's what makes it fun!