The amphibious airplane collided with power lines while on final approach to land on a lake. The pilot reported that he flew upwind over the lake at an altitude of 500 feet above the water to look for boats and obstructions. Not seeing any, he entered a traffic pattern to land. He reported that he was on final approach, slightly below tree top level, when the airplane contacted the power line. The pilot reported that he instantly applied full power and back elevator trying to keep the airplane level. He reported the airplane "pancaked" into the water. The pilot and passenger exited the airplane without injury. Inspection of the landing site revealed the poles which supported the power lines were located in wooded areas on both sides of the water and the color of the power line blended with the water. The pilot reported that residents who live on the lake informed him that the power lines were marked with orange balls until the lines were replaced a couple years ago at which time the balls were removed.