Closed Circle by Robert Goddard
The year is 1931. The new and luxurious transatlantic liner Empress of Britain is on her eastward passage. Among the first-class passengers on board are two English confidence tricksters, making a discreet exit from a little awkwardness they have left behind them in the United States. A chance meting on deck brings them a tempting new target in the shape of Miss Charnwood and her niece, the beautiful Diana, only child of the immensely wealthy Fabian Charnwood. It's a trick they've pulled before, with some success. Charm the daughter into an engagement to marry, then get the father to buy you off. So confident are they of success, in fact, that they make a pact: whichever of them wins Diana Charnwood's love will share his fortune with the other. Who would imagine that these smooth operators would let their hearts rule their heads? Or that violent death would find its way into their neat little scheme? Or that they would stumble into something much darker and deeper than either had suspected?