Death by Water - 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. Shipboard romances, champagne cocktails, erotic photographers, jealous swains, Mickey Finns, jazz musicians, blackmail, and attempted murder mingle before the thieves find out, as have countless love-smitten men before them, that where the glamorous and intelligent Phryne is concerned, resistance is futile.