film summary from wikipedia
A geology expedition in the Amazon uncovers fossilized evidence of a link between land and sea animals in the form of a skeletal hand with webbed fingers. Expedition leader Dr. Carl Maia goes to see his friend Dr. David Reed, an ichthyologist who works at a marine biology institute. Reed persuades the institute's financial backer, Mark Williams, to fund an expedition back to the Amazon to look for the remainder of the skeleton. They go aboard a tramp steamer called the Rita, which is captained by a crusty old salt named Lucas. The expedition consists of David, Maia and Williams, as well as Reed's girlfriend Kay Lawrence and another scientist named Dr. Thompson. When they arrive at Dr. Maia's camp, however, they discover that his entire research team has been mysteriously killed while he was away. Lucas suggests it was done by a jaguar, but the others are unsure. The audience is privy to the attack upon the camp, which was committed by a living version of the fossil skeleton the scientists seek.
The excavation of the area where Maia found the hand turns up nothing. Mark is ready to give up the search, but David suggests that perhaps thousands of years ago the part of the embankment containing the rest of the skeleton fell into the water and was washed downriver. Lucas says that the tributary empties into a lagoon known as the "Black Lagoon," a paradise from which no one has ever returned. The scientists decide to risk it, unaware that the amphibious "Gill-man" that killed Dr. Maia's assistants earlier has been watching them. It, taking notice of the beautiful Kay, follows the Rita all the way downriver to the Black Lagoon. Once the expedition arrives, David and Mark go diving to collect fossils from the lagoon floor. After they return, Kay goes swimming and is stalked underwater by the gill man, who then gets briefly caught in one of the ship's draglines. Although he escapes, he leaves behind a claw in the net, revealing his existence to the scientists.
Subsequent encounters with the gill man claim the lives of two of Lucas' crew members, before the gill man is captured and locked in a cage on board the Rita. He escapes during the night and attacks Dr. Thompson, who was guarding him. Kay hits the gill man with a lantern; driving him off before he can kill Dr. Thompson. Following this incident, David decides they should return to civilization, but as the Rita tries to leave they find the entrance blocked by fallen logs, courtesy of the escaped gill man. While the others try to remove them Mark is mauled to death trying to capture the creature single-handed underwater. The monster then abducts Kay and takes her to his cavern lair. David, Lucas, and Dr. Maia give chase to try and rescue her; ultimately she is saved and the beast shot. Riddled with bullets and stabbed in the heart by David's dive knife, the gill man stumbles into the water and is last seen sinking into the depths of the Black Lagoon.
The below summary information is from the MagicImage Filmbook "Creature From The Black Lagoon" production notes by Tom Weaver and from Donald F. Glut's "Classic Movie Monsters" chapter about the Gill Man movies, published by McFarland Books.
The Creature was the brainchild of producer William Alland. Recalling a story he was told by a South American movie director of prehistoric beings living somewhere along the Amazon River, Alland came up with the idea of a creature, half-man and half-fish. A missing link in the evolutionary chain that had survived unchanged by time. Alland chose Jack Arnold, who had just completed the 3-D movie "IT CAME FROM OUTER SPACE" to direct. Arnold met with Bud Westmore, the man in charge of Universal International's makeup department, and asked him to create a convincing Gill Man outfit. Westmore turned to his co-workers, and it took the talents of Jack Kevan, Chris Mueller, Milicent Patrick and Bob Hickman to come up with the final design. The first costume they made for test shooting was rejected in favor of the one everyone agreed was perfect.
Next, they needed a script and to find an actor to wear the Creature outfit. Maurice Zimm turned in a 59 page treatment which was fine-tuned by script writers Arthur Ross and Harry Essex. Glenn Strange, who had portrayed the Frankenstein Monster in the last few Universal horror sequels, was the studio's first choice to play the title role; he turned it down because he thought there would be too much swimming involved. Ben Chapman was told about the part and ended up signing a contract to play the Gill Man. He would play the Creature in all the scenes filmed above water. Ricou Browning has hired to do the stunt work underwater (and in fact did the underwater scenes in all three films.) Tom Hennesy took over in "REVENGE OF THE CREATURE" and Don Megowan was "THE CREATURE WALKS AMONG US." But unlike Karloff, Lugosi and Chaney, who became household names for playing the classic monsters of the 1930's and 1940's, the actors who played the Creature went uncredited. Universal International would have us believe that there really was a "Gill Man."