Final answer:
Children under 13 years of age must wear a U.S. Coast Guard-approved PFD when on deck and underway during boating.
Step-by-step explanation:
When boating, children under 13 years of age are required to wear a U.S. Coast Guard-approved Personal Flotation Device (PFD) when on deck and underway, according to U.S. Coast Guard regulations. This safety regulation helps to ensure the well-being of children participating in boating activities and is critical for preventing accidental drowning. It's important for all boaters to comply with this regulation for the safety of all passengers on board.