After a flight from Vancouver, British Columbia to Penticton, Canada, the pilot filed a flight plan for a return trip and departed at about 1900 MDT. Later, the aircraft crashed at night in the Little Bitterroot Lake, located about 25 miles west of Kalispell, MT. The pilot egressed from the plane, but the passenger was incapacitated after impact and drowned when the plane sank. The pilot left the accident site and was not located. However, he made 2 telephone calls (from New York and Texas) to a friend in Canada and indicated the location of the crash site. The aircraft was located in 255 ft of water and was recovered with the passenger. An examination of the aircraft revealed that the leading edge of the right wing and engine cowling were damaged during touchdown in the water. The right fuel tank contained 4 gal of fuel, the left tank was filled with water. No evidence of a mechanical malfunction or failure were found.