Fatigue failure of the vertical fin, accelerated by the installation of blind rivets in lieu of solid rivets in the replacement of the yaw SAS mount support, which resulted in the loss of helicopter directional control and collision with the terrain (water). Factors contributing to the accident were: a lack of information regarding repair of the yaw SAS mount support in the manufacturer's maintenance manuals, and the design of the vertical fin which was susceptible to fatigue cracking. An additional factor was the lack of an adequate passenger briefing.