NAVAL AIR STATION KEY WEST, Florida — A UH-60 Black Hawk helicopter hovered so low on the water during a recent demonstration that it created concentric white caps, seemingly clearing an area in the salty sea off Key West for divers to slip in. Once the divers in full-body camo dropped slender from the helicopter one by one, they disappeared below the water’s surface. And the chopper was off.

Armed and undetectable after six weeks of training at the Special Forces Underwater Operations School at Naval Air Station Key West, more than 1,000 divers annually are readied for global missions.