Miranda is a British movie made in 1948, starring a young Glynis Johns (who also played the mother in Mary Poppins and more recently Elsie in While You Were Sleeping). It's the story of a doctor who hooks a mermaid called Miranda while he's out fishing one day. She pulls him underwater into a cavern and proceeds to blackmail him – she will "let him off her hook" so to speak if he will show her London. He brings her back to his house, puts her in a wheelchair and disguises her tail with a blanket much to the chagrin of his wife. Trouble ensues when Miranda's flirty style gets all the menfolk to pay more attention to her than their wives and girlfriends. The incomparable Margaret Rutherford who is privy to her secret plays the nurse/mermaid-sitter.
The film was followed up by its sequel in 1954, which was called Mad About Men. Miranda the mermaid returns to land and swaps places with her cousin Caroline (a human – don't quite remember how that works) while on holiday and creates havoc in Caroline's love life.