The pilot reported that he loaded the airplane for a fire suppression mission with 450 gallons of water and calculated that the airplane's takeoff weight was 435 pounds below the maximum takeoff gross weight of 11,500 pounds. Using aircraft performance data, he determined that he would be able to perform the mission given the reported temperature and calculated density altitude. The airplane accelerated normally down the runway with engine instruments all showing full power indications. After liftoff, the pilot raised the flaps and the airplane accelerated to 85 knots. He expected the acceleration to increase to 105 knots during the climb-out, but the airplane did not accelerate normally, although all engine instruments showed full power. The pilot felt the airplane drop about 50 feet straight down, so he squeezed the jettison trigger to release some water. As the airplane continued to descend, the pilot released more water and maneuvered the airplane to avoid hitting houses and trees. Finally, the pilot pulled the nose back to stall the airplane in order to avoid hitting a house. The airplane clipped power lines, hit the edge of a house, and impacted the ground. The airplane sustained substantial damage to both wings, the fuselage, and the tail section. Postaccident examination found no mechanical anomalies with the airplane or engine that would have precluded normal operation. The pilot noted that he could have jettisoned the entire load of water when the airplane was not climbing, thus reducing the airplane's total weight by 3,800 pounds. This would likely have resulted in a positive climb rate.