The highly publicized accident, which happened as the ship was returning to Fort Lauderdale, at first skewered suspicion toward the child’s father amid claims that he had placed the child on a railing. But recent findings concluded the mother had committed an “irresponsible” act that led to the “life-threatening situation.”

Many other portholes on the ship the mother had taken photos with were blocked with glass barriers, and she assumed the openings on Deck 4 were similarly enclosed, the memo says she told investigators...