Undersea Cities (1954)
Rather than a floating city, today we have an image of undersea cities from the book Out of Time: Designs for the Twentieth-Century Future.
The book says that the image was published on the cover of if magazine in January of 1954. The most perplexing choice of the artist is why some of the "undersea cars" are driving on the ocean floor. It doesn't seem very practical but I'm sure that it helped 1950s audiences picture such an environment as familiar or more desirable.