The amphibian aircraft landed on relatively smooth water in a lagoon. As soon as the floats touched the water the aircraft slewed to the left, nosed into the water, and turned over. The aircraft came to rest upside down in about three feet of water. The occupants evacuated the aircraft within half a minute and the pilot activated an emergency beacon. A helicopter responding to the emergency arrived at the scene at about 0940. Visual inspection of the aircraft after the accident revealed that the left main gear was extended. The pilot stated that he had the correct gear-up indications before landing. An inspection of the aircraft did not reveal any reason for the extended left main gear. One passenger received minor injuries when fuel affected skin on her legs whilst standing in fuel contaminated sea water.