It was Karl Malden's idea to have his character speak in a different accent constantly. He thought it would be an amusing gimmick.
Unlike the novel, this takes a light-hearted, spoofy style with Dean Martin essentially playing himself. The Matt Helm of the novels is a much darker creation.
The large, white hovercraft appears to be a Saunders-Roe Nautical 6 (SR.N6). The smaller hovercraft appears to be the Britten-Norman "Cushioncraft" 5 (CC5). There was one CC5 built which capsized during tests off of Priory Bay.