The private certificated pilot was conducting a personal cross-country flight under Title 14, CFR Part 91, in a float equipped airplane. In an interview, the pilot said he received a full weather briefing, and filed a VFR flight plan, which included transiting a narrow mountain pass. He said prior to entering the mountain pass, three airplanes flying in the opposite direction exited the pass, leading him to believe it was open. The pilot said once inside the pass, the clouds closed in rapidly, and he lost sight of the river he was following. In a written statement, the pilot reported that the pass was "scuddy," and that he started a slow descent. Upon losing sight of the river he reported that he asked the pilot rated, right seat passenger if he could see the river. He wrote that the passenger, seeing a stream on the right, panicked, and said "turn right," and grabbed the controls, turning the airplane to the right. The pilot indicated that he regained the controls, and made a steep left turn, at which time the airplane impacted on a 45 degree slope. In his written statement, the pilot reported that he should have entered the pass at a lower altitude, and should not have been "scud running." He indicated that the accident would not have happened if the passenger had not panicked, grabbed the controls, and made the right turn. According to the pilot, there were no known mechanical anomalies with the airplane, and that the airplane received substantial damage to the wings and fuselage during the impact with terrain.