While you were out shopping Sunday for those last-minute holiday gifts, the Navy pushed ahead with its own vision of an underwater sugar plum: a fleet of “long endurance, transoceanic gliders harvesting all energy from the ocean thermocline.” Fact is, the Navy has been seeking—pretty much under the surface—a way to do underwater what the Air Force has been doing in the sky: prowl stealthily for long periods of time, and gather the kind of data that could turn the tide in war. The Navy’s goal is to send an underwater drone, which it calls a “glider,” on a roller-coaster-like...