It was a normal morning in April last year. Normal, that is, by the crazy standards of the fishermen, ship’s crews, navy sailors and Somali pirates plying their dangerous trades on 2.5 million square miles of lawless ocean stretching from India to Kenya.

“Dave,” a 44-year-old from Wiltshire in southwest England, was standing watch on the upper deck of a commercial car carrier bound from Mumbai to Mombasa. Scanning the horizon with a pair of high-powered binoculars, the former British Royal Marine of 24 years’ experience spotted something suspicious ahead of the carrier: a small freighter match...