LOS ANGELES—The captain of a scuba diving boat that burned and sank off the California coast, killing 34 people, pleaded not guilty Tuesday to manslaughter charges. Jerry Boylan was arraigned in federal court in Los Angeles on 34 counts of seaman’s manslaughter. Each count carries a potential 10-year prison term. Prosecutors say Boylan failed to follow safety rules before the fire broke out Sept. 2, 2019, on the Conception and led to one of the deadliest maritime disasters in recent U.S. history. Boylan was accused of “misconduct, negligence, and inattention” by failing to train his crew, conduct fire drills, and have a roving night watchman on the boat when the fire ignited. Boylan, 67, was indicted in December and surrendered for booking Tuesday morning. He was held in lockup and appeared in court by video wearing a blue surgical mask. He was expected to be released later on a $250,000 …