Mistaking large animals like manatees for the mythical creatures people usually ascribe sightings of mermaids to sea-weary mariners.
However, a new theory has suggested that they might have seen 'sea apes' instead.
A separate strand of primates evolved to live in the sea, according to a new documentary - and sightings of the 'sea apes' were described as mermaids.
"It's a very radical theory on human evolution, but we have approached an age-old myth and really chased its origins," the Daily Mail quoted Animal Planet's Charlie Foley as telling Fox News.
"It has been compiled in a way that is very compelling, making us think that mermaids might not just be mythical creatures," he said.
The 'aquatic ape' theory that a separate strand of primates evolved to live in the sea is regularly dismissed as pseudoscience.
Early hominins certainly lived near the sea and started sailing surprisingly early.
The documentary argues that as apes evolved into 'pre-human' hominins, some evolved to live in water.
The strand died out, but it is reported that for a time, there were aquatic ape-like creatures.
"There are striking differences between us and other primates, yet many features we share with marine mammals, like the webbing between our fingers, which other primates don't have, a layer of subcutaneous fat, and a loss of body hair," said Foley.
"We also have an instinctive ability to swim, and control over breath. Humans can hold breath up to 20 minutes, longer than any other terrestrial animal," he added.