While the documentary did perk up my interest, a few things weren't very convincing to me. I am no expert on species evolution and maybe I'm missing something, but wouldn't a species that adapted in such a way so as to even gain a tail, have also a pelvis with a similar adaptation? Or maybe it wouldn't even have a pelvic bone. The fact that that being had hips of a biped was a bit awkward to me. Following their logic in what concerns gravity, why would a marine being with a tail and fin still need a pelvis designed to support the weight of the organs when standing in an upright position, if he can no longer stand? Wouldn't an adaptational need have changed it the same way it had changed its two legs into a tail?
Then we have that short video of the two boys closer to the end of the documentary. I don't know, but it seemed at bit too much of a "oh look, that's a fin! Amazing...a pity we can't see the rest.." . Where is the rest? Wouldn't the boys have filmed more of that supposedly incredible being? I can't tell what happened, of course, but that did not convince me.
I wasn't too keen on the computer animations either. For me, they added an unnecessary sensationalist tone by pushing a conclusion on the viewer right from the beginning of the documentary.
I still don't know what to think of it though, maybe trues mixed up with fabrications? I don't know yet.
Documentary aside though, the fact that the mermaid legends have been around for such a long time and within so many different cultures can probably speak for itself, I think. Only a small percentage of the ocean has actually been explored, so the possibility of an hominid that has become water adapted can very well be real. The more I read, the more I find that what to me were previously incomprehensible myths, possible metaphors, or codes to certain sightings, seem to often be relatively accurate descriptions of very real events. We can take the example of plasma shapes created by electrical discharges, that can now be observed through controlled experiments, and that seemed to have been observed and reported through drawings in many ancient cultures all over the world. (there's a link with more detail about this but I wasn't able to find it)