The creature is usually regarded as the last of the great Universal monsters. The gill man became an icon for horror fans but never quite passed into the mainstream like Dracula or the Frankenstein creation. Still, this was a significant studio production with the underwater photography and its initial release in 3D.
There's the same story as King Kong (1933), though it keeps the part where the captive beast is shipped to America for the sequel. A scientific expedition to the Amazon uncovers a mysterious fossil, and then its descendant, a living creature which is half human, and half fish. Of course, it’s a man in a rubber suit, but still effective.
We get the kind of boffins who are best when seen in their swimwear. Richard Carlson and Richard Denning have a fistfight over the ethics of their mission. Julie Adams cavorts extensively in a water ballet while the gill man (understandably) watches approvingly. There’s the standard beauty and the beast theme.
It tries to exist in the creation myth of the bible and also the science of natural selection, which doesn't really compute, but at least work has gone into making the hokum halfway conceivable. There’s even some minor ecological subtext. All the classic motifs of the creature feature are delivered intact; but the clichés are still fun.