The captain of the Delta Queen decades ago was the rival of Colonel Beauregard for the affections of Magnolia. All of them are now deceased.
Magnolia was going to marry Beauregard on the Delta Queen, but she left him at the altar when she thought he had stolen her emerald necklace. In fact, the necklace was stolen by the captain of the Delta Queen, and hidden in the steering wheel of his ship in order to ruin the wedding.
The New Scooby-Doo Mysteries
Clues in a journal the captain left behind showed that he had stolen the necklace, and suggested where to look for it. Daphne Blakefound it.