The pilot of the amphibian airplane reported that she was flying a visual approach to a lake, over mountainous terrain. On an approximate 3/4 mile final approach to the lake, she observed a boat near her intended landing area and performed a go-around to circle for another approach. During the go-around and turn, the pilot did not move the throttle lever to full power. About one-fourth through the turn, she noticed the airplane was descending and increased engine power. The airplane continued to descend and she increased engine power a second time; however, one of the airplane's floats contacted a tree and the airplane impacted the ground. The pilot further stated that the accident could have been prevented if she had flown a higher approach and increased the throttle to full engine power immediately during the go-around. Examination of the airplane by a Federal Aviation Administration inspector revealed substantial damage to the wings and the fuselage. The pilot reported no preimpact mechanical malfunctions with the airplane and the inspector did not observe any.