Identify horror film/TV show where protagonists ostensibly hunting a Gill-Man are transformed into same

I am trying to track down the name of a film or TV show I saw as a child during the 70s, probably during a "Creature Feature" type TV rebroadcast. I believe the work was actually from the 50s or 60s (black and white, as I recall).

As I recall, there was some kind of crew of protagonists on a river-boat hunting for a Gill-Man/Creature From The Black Lagoon type of monster.

I also have a vague recollection that some of the protagonists/boat's crew vanish as the monster surreptitiously attacks across the film.

I have the impression that a (probably "the," and probably did a lot of screaming) sympathetic female character on the boat is kidnapped and the climax of the film reveals that the disappeared people have been/are being transformed in something like a surgical theater into piscene amphibious humanoids just like the primary antagonist.