THE FLT DEPARTED THE 34TH STREET HELIPORT ON A LOCAL SIGHTSEEING FLT TO THE STATUE OF LIBERTY & RETURN. AS THE FLT CROSSED LOWER MANHATTAN, ABOUT 1/4 MI FROM THE EAST RIVER, THE PLT HEARD THE LOW RPM AUDIO SIGNAL. HE LOWERED THE COLLECTIVE & ROTOR RPM RETURNED TO THE GREEN. WHILE TURNING TO THE EAST RIVER THE GENERATOR-OUT & ENG-OUT LIGHTS' ILLUMINATED. THE FLOATS WERE INFLATED & THE HELICOPTER CONTACTED THE WATER IN A LEVEL ATTITUDE. THE NOSE THEN CONTACTED THE WATER & THE CHIN BUBBLE BROKE. WATER ENTERED THE HELICOPTER & IT ROLLED RIGHT TO AN INVERTED POSITION. INVESTIGATIONDISCLOSED THAT THE FILTER LINE BETWEEN THE COMPRESSOR SCROLL & THE POWER CONTROL UNIT HAD BROKEN AT THE FILTER END. A VISUAL EXAMINATION OF THE FAILED PC LINE & THE B-NUT FITTING DISCLOSED THAT THE FRACTURE OCCURRED AT THE OUTBOARDEND OF THE FERRUL.