On August 1, 1993, about 1635 eastern daylight time, a float equipped Cessna 180 airplane, Canadian registration CFPTU, sustained substantial damage during a precautionary landing in Lake Michigan, approximately one-half mile off shore from Naubinway, Michigan. The Canadian certificated pilot was fatally injured; his wife, the sole passenger, received minor injuries. The flight operated under 14 CFR Part 91, and departed Shawano, Wisconsin, about 1420 central daylight time. A VFR flight plan was filed. According to statements made by the passenger, they had encountered deteriorating weather conditions while flying over Lake Michigan. They were flying in patchy to occasionally heavy fog, and her husband continued to fly lower to escape the fog. She said, in part, "With visibilities reducing we decided to land on the water near the shoreline. While landing the aircraft touched the water several times and then on the last touchdown it appeared to me that the right float dug into the water and then the aircraft cartwheeled... I am not sure of the height of the waves at the time except to say that when I surfaced the waves were washing over my head." The passenger told a Michigan State Trooper who interviewed her on the day of the accident that the airplane came to rest upside down, suspended by its floats with the cabin submerged in water. She said she released her safety belt, but could not get her cabin door open; her husband reached over and opened her window. With the window open, she was able to swim out of the airplane and grab onto a float. She was able to look into the water and see her husband half out of the window. He appeared to be caught on something in the cabin. She grabbed him but was unsuccessful in removing him. The passenger was rescued by a canoeist approximately four hours after the accident. Prior to his departure from Shawano, the pilot filed a VFR flight plan with the Green Bay, Wisconsin, flight service station, and requested the current and forecast weather for his destination (Sault St. Marie). He was advised by the flight service station specialist that a thunderstorm was currently active at Sault St. Marie, and that visual meteorological conditions were forecast for the pilot's projected arrival time. The pilot activated his VFR flight plan with the Green Bay flight service station at 1526, and reported to Green Bay at 1546: "would like to revise my en route altitude to three thousand five hundred feet, and I will stay very close inland 'cause there's storm activities in some sections of the lake...the inland is very clear in case other pilots inquire about the weather." During the investigation, it was learned from the Canadian Transportation Safety Board that the airplane's front seat occupant's seat belt restraint system had been modified with an after market shoulder harness. A portion of the pilot's shoulder harness and lap belt was recovered from the airplane by the Macinac County Sheriff's office and sent to the NTSB. The lap belt/shoulder harness portion received was an incomplete length of lap belt webbing with the male half of the metal latching mechanism, and a portion of the shoulder harness webbing with a "V" shaped clip fastened at the end. The "V" clip was secured to the male seat belt latch via a raised button. The "V" clip was readily removed from the raised button with minimal force. No manufacturer's name was on any portion of the passenger restraint system received by the NTSB. Two different serial numbers were found on the latching plate, 500950-405, and 500950. A conversation with the Macinac County Sheriff disclosed that the diver who had recovered the airplane found the seat belt portion of the restraint system unlatched, with the shoulder harness "V" clip still attached to the male latching mechanism. The diver reportedly told the sheriff that the male and female portions of the seat belt latching mechanism mated and demated without difficulty. An autopsy was performed by Doctors Carl Hawkins and Stephen D. Cohle, Blodgett Medical Center, Grand rapids, Michigan. The cause of death is listed as "Asphyxia by drowning."