On June 1, 2006, at approximately 0830 Pacific daylight time, a de Havilland DHC-3T (Otter) floatplane, N606KA, was substantially damaged during a precautionary landing in the Puget Sound near Port Townsend, Washington. The airline transport pilot and his 10 passengers were not injured. The flight was being operated by Kenmore Air Harbor, Seattle, Washington, under Title 14 CFR Part 135. Instrument meteorological conditions prevailed for the on-demand air taxi, international flight which had originated approximately 20 minutes before the accident from Kenmore Air Harbor Seaplane Base (W55), Seattle, Washington. The flight was being flown on a company activated visual flight rules (VFR) flight plan with a destination of Victoria, British Columbia. The pilot said that as he approached Port Townsend, Washington, weather conditions deteriorated with a lowering ceiling and visibility. He said that the water condition was glassy smooth with no visual definition. He said "this condition made it increasingly difficult to be sure I was maintaining forward visibility." The pilot said that he began to turn back to better VFR conditions, but conditions worsened; he believes that he inadvertently entered a fog bank. He elected to perform a precautionary landing on the water. He said that with no visual definition, he misjudged his altitude, and landed hard. He was unsure of the airplane's structural condition, so he taxied it to Port Townsend, Washington. The airplane's fuselage was bent/deformed, the fuselage skin was wrinkled, and the dorsal stabilizer was bent/wrinkled. At 0655, the weather conditions at Whidbey Island Naval Air Station, Washington, approximately 14 nautical miles north of the precautionary landing site were: wind calm; visibility 7 statue miles with shallow fog; cloud condition 900 feet scattered, 2,000 feet scattered, 5,000 feet broken and 20,000 feet broken; temperature 55 degrees Fahrenheit; dew point 54 degrees Fahrenheit; altimeter setting 29.95 inches. The pilot reported that at the time of the attempted landing, the visibility was zero, the wind condition was zero, and the ceiling was zero.