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. The Gill-Man suit is interesting because it's one of the few monster costumes of that era that was designed for long periods of underwater photography. The scenes where the Gill-Man slowly stalks the love interest through the waters she's swimming through are wonderfully suspenseful.
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 lackluster sequels: "Revenge of the Creature" and "The Creature Walks Among Us".
Some research from the Internet Movie Database (www.imdb.com)