On July 26, 2010 about 1442 central daylight time, a Bell 206L4 helicopter, N317RL, had a loss of main rotor rpm and the pilot executed an autorotation to the water about 63 miles southwest of Houma, Louisiana in the Gulf of Mexico (GOM). The commercial pilot and his two passengers were not injured. The helicopter was registered to and operated by Rotorcraft Leasing Company LLC, Broussard, Louisiana. Visual meteorological conditions (VMC) prevailed in the vicinity and a company flight plan was filed for the Title 14 Code of Federal Regulations Part 135 on-demand passenger flight. The helicopter had just taken off from the South Timbalier 178A offshore oil platform and its intended destination was another nearby offshore platform. The pilot reported that during takeoff from the platform he heard what seemed to be loud banging from the rear of the helicopter and noted a loss of main rotor rpm. The pilot activated the emergency float system and initiated an autorotation to the water. Upon touchdown, the engine was still running. The pilot shut down the engine and prepared the passengers to evacuate. All three occupants safely evacuated the helicopter (which was upright on its skid mounted float system) and boarded the emergency life raft that the pilot had inflated. A nearby surface vessel came to their aid and transported the three occupants to a nearby platform. A Coast Guard helicopter later transported them to an on-shore location for a medical evaluation. The helicopter remained upright floating on the water and was later recovered and transported to the operator’s on-shore maintenance facility. The operator did immediate fuel quality tests on the facility where the helicopter had most recently been refueled and found no problems. An examination of the helicopter drive systems did not reveal any preincident anomalies which would have precluded normal operation. The engine was removed intact and was operated within normal parameters during an engine test cell run. The reason for the pilot’s report of a partial loss of main rotor rpm could not be determined.