Mystery Aboard the Ocean Princess by Robin Gottlieb, 1967.
Jill Washburn is about to make her first trip to Europe on the same ship as a priceless painting! Her Uncle Chow and Aunt Rose have invited her to accompany them on the cruise ship to Europe over the summer because her cousin Connie isn’t enthusiastic about the trip, and they thought that the girls would be good company for each other. Lately, Connie has been trying to act ultra-sophisticated, declaring anything and everything that she doesn’t think is cool to be simply “tedious” (Connie loves big words, and “tedious” is her pet word).
Jill is fascinated by the presence of the famous painting on the ship, a painting of a young Spanish prince by Goya, “Don Manuel Osorio de Zuñiga.” Jill loves art, and her ambition in life is to be an art teacher, like her father. She has also read about recent art thefts in the newspaper and wonders whether Don Manuel is at risk during its journey. She can’t help but seek out the room where the painting is kept and talk to one of the guards about security. The guard just tells her that he appreciates her concern but that everything is under control. But later, Jill finds the guard meeting secretly with a mysterious man by the lifeboats.
Could the two men be plotting to steal the painting? Jill persuades Connie to help her investigate and spy on the them, but things aren’t what they seem.
A mysterious gift left in Uncle Chow and Aunt Rose’s cabin frames Jill’s aunt and uncle for art theft. Although Connie thinks their snooping has caused enough problems already, Jill thinks that the only way to clear their names is for her and Connie to find the real thieves. With the ship’s costume ball coming up, Jill comes up with a daring scheme to get the thieves’ attention and hopefully get them to reveal themselves. One thing is for sure: Connie was wrong about this trip being “tedious.”