The U.S. Navy’s Diving and Salvage Center has been training highly skilled deep-water divers for decades who, once assigned to American military units worldwide, are capable of technical diving to do the good—using C4 explosives to clear harbors and beaches of debris and unexploded ordinance—as well as the bad, like blowing up foreign oil rigs, fouling intake valves for undersea power plants, destroying locks on crucial shipping canals.