This, the earliest of Hammond Innes's sea stories is a fairly typical WWII adventure story, with a pair of fairly ordinary Englishmen thwarting the nefarious designs of Nazi spies. It was written, however, during the very earliest weeks of the war and is extraordinarily prescient; the threat to shipping from submarine warfare was known, but U-boat pens did not exist until some years later.Innes also scores extra points (from me) for including a plucky and intrepid female journalist investigate the mystery and for the Cornish setting. I would point out that Cornish wreckers only seem to have ever existed in fiction, but can't complain of him using the idea.
What do You think about Wreckers Must Breathe (1995)?