If it weren't for the fact that the two films came out around the same time in different countries, I'd say the other either copied the British Miranda or the USA's Mr. Peabody and the Mermaid. Both William Powell and Griffith Jones go fishing and get themselves a fetching young mermaid for their troubles.
Ann Blyth's Lenore in Mr. Peabody is a silent young lady, but that's definitely not true of Glynis Johns in the title role of Miranda. She not only talks, but is rather well read on human kind. Of course one does not learn everything from books.especially some of the interspecies facts of life.
Because it's a man that Miranda is seeking. The mermen of the oceans seem to be just not her type. And though Griffith Jones is married he is taken with her.
Being a doctor Jones concocts a wild cover story involving her being a paraplegic patient who has come to live with him and wife Googie Withers. He has dresses made an extra foot long to cover her tail and has her in a wheelchair for the most part. And she's on an exclusively fish diet. Unusual doctor's orders to say the least.
Miranda is a nice little fantasy and Glynis Johns practically glows in the title role. Besides Jones and Withers other performances of note are David Tomlinson as their butler and the irrepressible Margaret Rutherford as the practical nurse that is engaged.
Hopefully it will come out on DVD/VHS and soon.