Carter Dickson (what John Dickson Carr called himself in his locked-room Sir Henry Merrivale mysteries) created a good setting for a wartime mystery: an ocean liner running munitions for the English in early 1940. Steaming through a storm, its crew alert for u-boats, caring a half million pounds of explosives, and with a shady group of passengers (more than one of the passengers gets murdered, after all, out of a population of nine), the SS Edwardic has the plotting advantage of providing one large locked room in the shape of the ship itself.
The writing (Merivale's dialogue excepted) is more than competent, but the execution of the plotting has flaws, and the characters (the young woman aside) have quirks without having memorable personal qualities. Merivale, like Gideon Fell, is all bluster. When he detects something, we're not in on how he did it. The setting, which promises much, isn't used to full advantage; e.g., blackout conditions reduce much of the action on deck at night to something out of a Marx Brothers film, minus the humor.