On July 13, 1995, at 1830 Alaska daylight time, an amphibious float equipped De Havilland, DHC-3 airplane, N472PM, registered to and operated by Taquan Air Service of Ketchikan, Alaska, experienced an in-flight separation of a portion of the elevator servo tab during cruise flight near Ketchikan. The on demand air taxi flight, operating under 14 CFR Part 135, departed Ketchikan for a local flight with an intermediate stop at Clover Bay, Alaska, which is located on Prince of Wales Island. A company visual flight rules flight plan was in effect and visual meteorological conditions prevailed. The airline transport certificated pilot and the seven passengers were not injured and the airplane received substantial damage. According to the chief pilot, the pilot had leveled off for cruise flight and was trimming the controls. While trimming the elevator servo tab, the airplane began vibrating and the pilot landed the airplane on the water near Ketchikan. Subsequent examination of the servo tab showed that the tab was torn in half approximately mid span. The tear started at the trailing edge and progressed forward along the chord of the servo tab. The portion of the servo tab with the control rod attached remained with the airplane. Examination of the servo tab showed small cracks starting from the trailing edge and moving forward toward the front of the servo tab. Two of the cracks are located near the separation tears and the other two cracks are located in the undamaged free span of the trailing edge. According to the Director of Maintenance the servo tab was replaced and the rigging and trim assembly was inspected and no problems were found. During a telephone conversation with the Director of Maintenance, he stated that the airplane's tail is approximately 10 to 11 feet above the ground and is very difficult to examine during preflight. He felt that there was previous damage to the servo tab at the point that it tore/separated. The company did not submit passenger names or address on the NTSB form 6120.1/2.