Creature From the Black Lagoon

Creature From the Black Lagoon (1954) is one of my favorites of the old monster movies. Maybe it's that music...

trailer:

Empire Online says,

The underwater scenes remain definitive, with the curvy Adams floating on the surface and dangling her long white legs above the Creature's claws, and the Gill Man performing a serpentine underwater ballet beneath her pin-up form. Few 1950s science fiction films bothered with sex, but this swimming flirtation remains as classic an image of impossible love as King Kong and his tiny blonde.

The Hollywood Reporter in a review from the time of the film's release calls it "a good piece of science-fiction of the beauty and the beast school, the beast in this case being a monstrous combination of man and fish. It makes for solid horror-thrill entertainment." Rotten Tomatoes has a critics score of 84%.