THOUSANDS OF PIECES of pottery like this one—crafted in Campania, in what’s now southern Italy—filled the hold of a wrecked Greek ship. The cargo went down in the western Mediterranean, only to rise again a couple of millennia later thanks to Jacques-Yves Cousteau, famed oceanographer and National Geographic Explorer. In 1952, off a French islet near Marseille, his divers launched one of the world’s first underwater excavations, and the breadth of the haul was extraordinary. Alongside more than 7,000 ceramic pieces were some 2,000 clay jugs, called amphorae, once full of wine. (They’d belonged, archaeologists later realized, to two different ships wrecked at the same site.) When Cousteau found a single jug still sealed, he had the cork scraped away and poured out what a crewmate called a “dark brown,…