The Disco Volante was indeed Largo's motor yacht but it first appeared in Thunderball of which Never say Never Again was a disappointing remake featuring little of the style or flair of the original. The greatest star of the re-hash was undoubtedly Sean Connery's toupe which had important supporting roles in several of his films in the 80's.
The Disco Volante was able to raise out of the water on a hydrofoil which gave it a great speed advantage over a regular displacement design. She also had a split hull allowing the front part of the vessel to detatch from the rest of the ship to facilitate speedy escape.