The Last One Left — John D. MacDonald

When a yacht explodes in the Bahamas, apparently killing six people, Sam Boyleston, an attorney from Texas and the brother of one of the victims, is compelled to investigate the circumstances, as does Raoul Kelly, a newspaper reporter.

After the disaster the yacht's burned captain was temporarily marooned on a small island, and soon it becomes apparent that one person is ruthlessly manipulating events. But for Boyleston and Kelly proving guilt appears impossible ...

'A major suspense novel' New York Times