While approaching the pontoon, the pilot reported noticing a warning message illuminate but was unable to verify the nature of the warning due to workload. Pilot elected to conduct a go-around. On the second approach at an altitude of about 40 ft above the pontoon, the pilot recalled feeling a ‘thud’ and the nose of the helicopter yawed sharply left. The pilot was unable to regain control of the helicopter and it collided with the water. The pilot and two passengers exited the helicopter soon after the collision and swam free from the upturned helicopter. The remaining rear passenger was evacuated by one of the surviving passengers and the front passenger (in the copilot seat) was evacuated from the helicopter by the pilot. The two passengers that were unable to exit the helicopter unassisted were fatally injured. The two other passengers sustained minor injuries, and the pilot was uninjured.