Classic horror movie, released in 1954. It was directed by Jack Arnold and written by Harry Essex and Arthur A. Ross, based on a story by Maurice Zimm. It starred Richard Carlson, Julie Adams, Richard Denning, Antonio Moreno, Nestor Paiva, Whit Bissell, Ricou Browning (as the Gill-Man in the water), and Ben Chapman (as the Gill-Man out of the water).
Basic plot: A scientific expedition on the Amazon River stumbles upon a "Gill-Man" -- the missing link between humans and fish -- living in the mysterious Black Lagoon. The creature is captured, but escapes to take the heroine back to his swingin' bachelor pad.
Originally released in 3-D, this movie has really beautiful underwater photography and outstanding makeup.
Best recommendation: my sister, who normally dislikes horror movies -- especially old horror movies -- watched this all the way through a few years ago on AMC, jumped in all the right places, and declared afterwards that she'd watch it again. Score one for the Creature!
It was followed by two sequels: "Revenge of the Creature" and "The Creature Walks Among Us".
Some research from the Internet Movie Database (www.imdb.com)