The Corsican Shadow (Dirk Pitt 27) by Clive Cussler

From the shadow of war comes an enduring historical mystery - and a present-day plot of destruction . . .
France, May 1940. As the German army blitzes Europe and Parisians flee their city, the chief curator of the Musee de l'Armee is ordered to take a mysterious piece of cargo out of the country. But when he learns his intended ship has been sunk, the fate of his cargo must rest on a decrepit steamer, sailing out under German fire. . .
Decades later, a diving expedition in the English Channel leads Director Dirk Pitt to a cache of uncut diamonds on a shipwreck. No sooner than they are discovered, the diamonds are stolen! Pitt and the NUMA agency now find they must face a murderous cabal with far more destructive plans than mere theft. Vital water treatment facilities around the globe are being targeted - placing the world's population in grave peril.
From the shadow of the Eiffel Tower to the islands of the Caribbean, only Dirk Pitt and his children, Summer and Dirk Jr., can locate the treasure that will save the world from catastrophe.