In 1942 members of the Royal Marines Boom Patrol Detachment led by Major 'Blondie' Hasler used the sea off Southsea and Eastney to practice for a daring raid on enemy shipping in France. Their task was to canoe 70 miles up the River Gironde to Bordeaux and use limpet mines to sink a number of cargo ships reported to be preparing to take the latest German fuses, radios and radar equipment to Japan. The type of canoe chosen for the raid was the Cockle Mark 11 - hence the name of the book and film. 'The Cockleshell Heroes' which have been made the raid famous.
 
Six canoes and their two-man crews were transported to French coastal waters by the submarine 'Tuna' in December 1942. One canoe was damaged as it was being brought through the submarine's hatch and could not be launched. The other five canoes started their hazardous journey but two quickly capsized in the tidal races at the mouth of the River Gironde. Another canoe sank near Bordeaux leaving two crews to make the final attack which resulted in four ships being badly damaged. Major Hasler and Marine Sparks made it back to England with the help of the French Resistance but they were the only survivors. One crew member was drowned while another probably met the same fate. Six marines were captured by the Germans and subsequently shot in accordance with Hitler's directive: 'All enemy troops taking part in the so-called commando operations...whether in battle or escaping...will be destroyed to the last man.'