THE BELL 206B WAS ON A SIGHTSEEING FLIGHT AROUND THE ISLAND OF MANHATTAN WHEN IT EXPERIENCED A LOW ROTOR RPM SITUATION. THE PILOT SAID HE COULD NOT RECOVER ROTOR RPM AND MADE A FORCED LANDING IN THE EAST RIVER. THE PILOT AND THREE PASSENGERS EXITED THE AIRCRAFT AND HUNG ONTO THE FLOATS WHICH WERE INFLATED AND SEPARATED FROM THE AIRCRAFT. ONE PASSENGER DID NOT ESCAPE, WAS FOUND IN THE AIRCRAFT 1 1/2 HOURS LATER AND DIED OF DROWNING. A FATIGUE CRACK WAS FOUND ON THE POWER TURBINE GOVERNOR NEAR THE PY PORT WHICH WOULD CAUSE THE ENGINE TO LOSE POWER. THE PILOT AND PASSENGERS SAID A WARNING HORN IDENTIFIED AS THE LOW ROTOR WARNING HORN, STAYED ON THROUGHOUT THE DESCENT. THE LOW ROTOR RPM WARNING HORN IS DESIGNED TO SOUND WHEN A LOW ROTOR RPM CONDITION EXISTS. NO EVIDENCE OF ANY COLLECTIVE CONTROL PROBLEM WAS FOUND AND THE PILOT DID NOT MENTION A JAMMED COLLECTIVE.