SEATTLE - Five hundred miles north of Alaska, a group of shipmates from the Coast Guard cutter Healy tossed a football on the blue-and-white, diamond-hard Arctic ice. Others snapped panoramic photos and took walks during the two-hour break, stretching their legs after a month aboard the 420-foot icebreaker. Lt. Jessica Hill and Boatswain's Mate Steven Duque seized the chance for a training dive and slipped into a patch of open water near the Healy's bow. A team held ropes attached to the divers, lest they become disoriented under the ice. Several research scientists watched from the deck. But no one...