Cryptozoologists sometimes speculate that behind the stories of mermaids and mermen there might be (or might have been, at one time), some kind of sea mammal with an oddly human appearance. It seems unlikely, though, especially because of the mermaid sightings stories (which, incidentally, do persist until the present day) where the upper half of the creature is almost indistinguishable from a long-haired Caucasian female. (The only difference that sometimes gets reported is the existence of taloned hands.)
Any mammal that has evolved into an aquatic lifestyle is simply not going to look like a terrestrial human because the environments and adaptations are different. Even if you assume a formerly terrestrial primate somehow evolved to have a tail, the upper half would not look human. It would look somewhere between the ancestral primate and the known aquatic mammals. The long hair in particular would be pointless. The same is true even if you subscribe to the highly speculative (and, in my view, not credible) Aquatic Ape Theory (AAT) that our own ancestors we through a seagoing phase. An ape or even a primitive human that stayed with the marine lifestyle would not present the appearance of a modern human.
I think it most likely these stories are what most scientists think they are: some honest misidentifications or seals or manatees plus some deliberately exaggerated accounts of same.