The film was set in the 50's or 60's about a grifter that a wealthy man befriends. The grifter murders the man on his own boat and throws him overboard.  The local gendarme does not believe the grifter is for real (the wealthy man was a recluse), and he persues the grifter until  he is satisfied the man is who he says he is...until the end of the film of course.  I even remember a scene where the grifter has gotten a copy of the wealthy man's signature, has it blown up so that he may perfect his forgery.  I saw this film as a pre teen many moons ago, and I never forgot it!  I would really appreciate it if anybody could tell me the title.  Thanks!