The pilot stated that the purpose of the flight was for the two passengers, who were both biologists, to conduct a low-level river/land survey. Prior to departure, the pilot discussed the route of flight with the passengers and they all reviewed the maps and sectionals for possible hazards with an emphasis on wire crossings. After departure, the pilot attempted to facilitate the biologists' view out the windows by maneuvering the helicopter in a crab angle with the flight path tracking down the riverbank and the nose pointing left. The pilot maintained an altitude just above tree-top level and about an hour into the flight the pilot observed the first high power line crossing. Approximately 20 minutes thereafter, one of the passengers called attention to another line crossing, which was equipped with marker balls. The pilot stated that as they continued, the river divided into numerous channels and one of the passengers advised him which path to follow. After surveying one of the channels, he maneuvered the helicopter in a 90-degree turn to circle back and then it was decided they did not need to return to the channel's origin. The pilot then made a left turn to adjoin the river and immediately observed blue sparks and a wire contact on the upper windscreen bubble. The helicopter was substantially damaged after it descended and came to rest in 4.5 feet of water about 50 feet from the power line. The pilot reported no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation.