Death By Water by Kerry Greenwood

The nice men at P&O are worried. A succession of jewelry thefts from the first-class passengers is hardly the best advertisement for their cruises. Especially when it is likely that a passenger is the thief. Phryne Fisher, with her Lulu bob, green eyes, cupid's bow lips, and sense that the ends justify the means, is just the person to mingle seamlessly with the upper classes and take on a case of theft on the high seas - or at least on the S.S. Hinemoa, on a luxury cruise to New Zealand. She is carrying the Great Queen of Sapphires, the Maharani, as bait.