It looks from that SPR photo that they had dummy landing craft actually on the beach. According to the film website, they had some genuine LCVPs for the at-sea shots, and used an LCM to carry the actors -- I suppose they needed room for cameras and stuff. Looking at the pictures, and the relative heights of the figures and the hull sides, I can believe they used an LCM as a floating set. And I'd agree that those landing craft on the beach are a bit crudely done. Not even any markings?