The pilot reported that he was departing from a small remote lake in a float-equipped airplane. He said that during the initial climb he encountered a strong downdraft, and he selected an area of marshy, brush-covered terrain as a forced landing site. During touchdown, the left wing collided with a stand of tall brush, which pivoted the airplane 90-degrees to the left, and the airplane's left float subsequently struck the shoreline. The airplane sustained substantial damage to the left wing. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.