THE HELICOPTER BEGAN AN UNCOMMANDED RIGHT TURN WHILE FLYING OUT OF GROUND EFFECT AT ABOUT 20 KNOTS FORWARD AIRSPEED WITH A DENSITY ALTITUDE OF 6,875 FT. THE PILOT REPORTED THE WINDS TO BE FROM THE HELICOPTER'S THE RIGHT FRONT WHEN IT STARTED THE UNCOMMANDED RIGHT TURN. THE PILOT ATTEMPTED TO ARREST THE RIGHT TURN BUT WITHOUT SUCCESS. THE HELICOPTER DESCENDED UNCONTROLLED UNTIL IT CRASHED INTO THE LAKE. THERE WERE NO REPORTS OF ANY PREEXISTING MECHANICAL FAILURES OR MALFUNCTIONS THAT MAY HAVE CONTRIBUTED TO THE LOSS OF CONTROL. THE PILOTS OPERATING HANDBOOK (POH) WARNS PILOTS TO EXERCISE CAUTION WHEN OPERATING AT LOW AIRSPEEDS AT HIGH ALTITUDES. THE POH INDICATES THAT TAILROTOR CONTROL MAY BE MARGINAL WHEN THE RELATIVE WIND IS IN THE CRITICAL WIND AZIMUTH AREA WHICH BEGINS AT THE RIGHT FRONT OF THE HELICOPTER. THE PILOT FAILED TO EVALUATE THE HELICOPTER'S PERFORMANCE DATA BEFORE DEPARTING.