The movie in question is the 1979 British movie entitled "The Riddle of the Sands". Based loosely on a novel, written in 1901 by Erskine Childers (who was later executed by the British during the Irish Revolt). I read the novel years before the film and highly recommend that you do the same. Like most cases, the original book beats the movie hands down for suspense. After all for 1901 this must be if not the first spy thriller, among the first. A gripping tale of sailing, espionage and flirtation. But don't take my word for it. Get your hands on a copy of the book or if you're a slow reader then view the film itself; the movie features Michael York in the lead role:
Tillerman: you can wire me the huge prize. I'm confident that this is the winning entry.