The pilot advised that the weather was fine, water conditions calm, and the river running at 6 - 8kt. The aircraft was launched from the bank and taxied downsteam for departure. The pilot reported that the aircraft's acceleration, and climb onto the step appeared to be normal, but before the aircraft became established on the step it began to porpoise. He attempted to overcome the oscillations by applying forward elevator control pressure which only aggravated the situation. He then tried to find the optimum position on the step by raising and lowering the nose with the elevator control, resulting in him chasing the oscillations. The aircraft eventually "wheelbarrowed" on its bow, preventing it from accelerating to its safe take-off speed. The pilot persisted with the take off, and while attempting to turn a bend in the river water pressure on the bow damaged the left nose gear door, creating an increase in water drag. This caused the aircraft to water loop to the left, damaging the left sponson and rear fuselage. The pilot rejected the take off and taxied the aircraft to the shore.