The miniatures were filmed using a real horizon in Santa Barbara harbor. Byron Haskin who was the head of the special effects department at Warner Brothers, mentions in a Directors Guild of America Oral History book, interviewed by Joe Adamson, that they strung wires across from Stearns Wharf to the shore at the yacht club with which to control the model convoy. The miniatures were built to 1 inch to the foot or 1/12 scale. He states there were as many as 450 separate miniatures used in the show. He goes on to say that there was a man in each ship controlling the steering of the small inboard motors. I suspect that the wires were for the background ships while the motorised ones were carving up the waves in the foreground. Jack Cosgrove was the supervisor for the miniature effects shoot.
The model work is really spectacular and always full of vigorous movement. There is a lot going on in the shots with a large number of ships in each frame of the convoy scenes. All the explosions are enhanced by a violent jarring and shaking of the camera blurring the footage which is quite effective. This effect was applied later by use of the optical printer.
All this adds up to it being one of all time great model ship movies.
