The pilot reported that the airplane's takeoff gross weight was 300 pounds less than the maximum. He performed a short-field takeoff from the 1,800ft gravel surface, and the initial climb continued as he expected. He recalled that the climb performance degraded and became insufficient to clear the 80ft tall trees located about 1,000ft from the departure end of the runway. The manufacturer's Pilot Operating Handbook (POH) stated that, short field takeoff's that require obstacle clearance will, after breaking ground accelerate to best angle of climb speed, 95 MPH and climb past obstacles. The airplane collided with treetops, descended and subsequently impacted the lake and became submerged. The airplane sustained substantial damage to the stabilator. Additionally, the POH denotes that the minimum runway distance required for the airplane to takeoff over a 50ft barrier was 1,360ft. According to the manufacturer, the minimum runway distance of 1,360ft represents a takeoff from a hard surface runway. Further, the POH specifies the use of a soft-field, obstacle clearance takeoff, as the increase flap setting of 25°, airspeed acceleration and pull nose gear off the ground as soon as possible and at the lowest possible airspeed. Accelerate just above the ground to best angle of climb speed, 95 MPH to climb past obstacle clearance height. Continue to climb while accelerating to best rate of climb, 105 MPH while slowly retracting flaps. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.