The helicopter was on a 14 CFR Part 135 air tour flight, which was scheduled to tour around the island of Maui. The pilot made the decision to tour the island of Molokai when weather along his scheduled route deteriorated. He did not inform the company of the routing change. While in a hover approximately 150 feet from a shore line, rotor RPM deteriorated and the inflatable float-equipped helicopter made an emergency landing in the water. The floats were deployed as the helicopter entered the water; however, the landing was hard and the helicopter sustained substantial damage. (To deploy the floats, the pilot was required to remove his hand from the collective control.) The pilot and five of the passengers were not injured. The remaining passenger received serious injuries. After landing, the seven occupants donned life vests and swam to the shore where they spent the night before being located by airborne search personnel. The helicopter drifted ashore and was examined. No evidence of a preimpact mechanical failure or malfunction was found.