ABOARD THE USNS SALVOR, Yellow Sea — Clear skies and a billowing sea Saturday were not telling of what was beneath the surface: brutally strong currents, frigid temperatures, and water so murky that divers would barely be able see in front of them if they had to go in. Those conditions, U.S. Navy divers and officials aboard this rescue and salvage ship said, are expected to persist and will make salvaging the wreckage of the South Korean patrol ship Cheonan that mysteriously sank more than a week ago unusually difficult. “This is a very challenging dive scene,” said Capt. Charlie...