The barge struck a disabled amphibious "duck boat" on July 7, killing two Hungarian students and plunging 35 other people into the Delaware River.
The mate, who was not identified, told a tugboat company lawyer he had learned that day of a life-threatening medical emergency involving his young son, the National Transportation Safety Board report said. He made or received 21 calls on his personal cell phone from the time he took the wheel at noon until the crash at 2:37 p.m.
The tug boat company had a policy in place that crewmembers should not be using cellular phones while on duty, according to an NTSB report.
The 4,400-page report released Monday also said the mate did not assign a lookout on the high-sitting barge as it was being pushed from behind by the small tug.
A federal criminal investigation is also under way. At his lawyer's advice, the mate has declined to cooperate with NTSB investigators, citing his Fifth Amendment right to avoid self-incrimination.
Ride the Ducks, the tour boat company, has not resumed its land-and-water tours in Philadelphia.
The Associated Press contributed to this story.