I think that they are a result of myths of water sprites combined with overlong sea voyages.

A parallel may be drawn to the Yeti; the name in itself is meaningful only to a restricted geographical area. Therefore, the idea of the Abominable Snowman was used. The “Snowman” here obviously was not intended to describe its appearance – it can be assumed that it meant something like a “man” which lives in the ice and snow-covered areas of the Himalayas. However, the majority of persons internalizing the description would have no idea what an actual Yeti looked like, so their brain would automatically link to the known concept of a snowman – worse yet an idyllic winter scene of children playing outdoors decorating their snowmen with hats, carrot noses and coal buttons.

While unintended, the use of a specific word in the description becomes entrenched. Like in the case of Merbeings then, our descendents would be highly confused in trying to figure out how a Yeti siting (if they still exist) could be related to a “Snowman” and would be very likely to reject the idea that it could be the same thing. Another example would be a description of an observation as something “moving across the sky in a manner similar to a saucer being skipped across the lake” which unfortunately became a “flying saucer” (forgive this ufology reference, but it is a very relevant example).

So, to return to the classical merbeings – it is very possible that a proto-sailor (searching for a way to describe something he had seen while out at sea) tried to convey an idea along the lines of “something like a fish that was almost human in its behaviour”. So, the idea (and depiction) became a half-fish / half-human creature that probably looked nothing like what the restricted group of ancient seafarers had actually seen. The Manatee seems unlikely not only for its obvious differences to pop-culture mermaids, but also its total difference to a fish (I have never heard seals, sea lions, walruses etc being compared to fish). Therefore, if it really was a marine mammal, I suspect DOLPHINS – they are known to follow boats (and guide them back to being within sight of land), and in the early days would have probably been even more curious about the land-mammals (humans) and their crafts. While dolphins do not look human or even like classical mermaids – the sailor would have been at a loss to describe this “almost human fish-like being” and therefore by having to reference known ideas, the Merbeing became fixed in the minds of the people (the majority of whom would know them only from accounts of sailors).