The Dark Ocean
Betty Haverhill leaves San Francisco on an Italian freighter bound for ports in El Salvador, Panama, Venezuela, Spain and Italy. Among the passengers is Mik Finsch, a mysterious man-of-the-world. Betty's mild flirtation with Mik ends in a fight and the suicide of one of the passengers. Another "suicide" leaves no doubt in Betty's mind that Finsch is a murderer, and soon she's fighting for her own life.
- summary from the official Jack Vance website