The local scenic flight departed from a lake in a southeasterly direction and the wind was reported at 8 knots from the south-southeast. Shortly after becoming airborne, while approximately 100 feet above ground level, the pilot noted that the airplane's airspeed was not increasing as he expected and that the airplane was not climbing. In an attempt to increase airspeed and ultimately gain altitude, the pilot lowered the nose but the airspeed did not increase. Due to terrain ahead of the airplane, the pilot attempted a landing on the remaining lake area. During the attempted landing, the airplane's floats contacted terrain, which was located between the main lake and a bay, and the airplane nosed over. A postaccident examination of the airplane showed substantial damage to both wings and the fuselage; no airframe or engine anomalies were noted. The pilot estimated his weight and balance at the gross weight limit and reported that he could have increased his safety margins by using all available back-taxi distance and more conservative abort points.