The Cessna 206 floatplane was on a charter flight taking passengers from Kimberley Coastal Camp to Lake Kununurra. During descent 25 NM from Lake Kununurra, the aircraft's engine lost power. The pilot reported that he immediately switched the fuel selection from the left to the right fuel tank. He also selected full throttle, full rich fuel mixture, boost pump to low prime and full fine on the propeller pitch control. The engine did not respond so he elected to conduct an emergency landing on the Ord River. The pilot reported that the descent took about five minutes, during which he selected high prime on the boost pump for several seconds. The engine did not recover power throughout the descent. The landing was conducted without further incident. The aircraft was undamaged and the passengers were uninjured. The pilot reported that he filled the tanks before leaving Lake Kununurra for the Camp. Because of the amount of fuel he thought was in the left tank, the pilot had intended to select the right tank at 15 NM from Lake Kununurra. However the engine lost power before he could do so. After the landing, the aircraft's left fuel tank was found to have about 8 L of fuel remaining, approximately 20 L less than the pilot had planned. The right tank had about 60 L. The operator reported that the only fuel system fault was three clips that secured the left fuel tank bladder to the wing were found undone. No fault could be found with the engine.