On February 25, 1998, about 1455 eastern standard time, a Lake LA-4-200, N3020P, registered to and operated by Sea Flight, Inc., as a 14 CFR Part 91 personal flight, crashed while attempting a water landing on Lake Murray, South Carolina. Visual meteorological conditions prevailed, and no flight plan was filed. The aircraft sank and was substantially damaged. The private-rated pilot and two passengers were seriously injured. The flight originated from Columbia Metropolitan Airport about 27 minutes before the accident. Following the accident, the pilot stated he thought that either the nose gear had inadvertently extended or that during initial water touchdown, the hull impacted a submerged object. Witnesses on the lake were mixed in their individual observations of whether the landing gear were up or down during the water landing. An eyewitness, the wife of a Lake amphibian owner and an ex-flight attendant, stated that she observed the airplane fly over her lakeside house and that all landing gear were up. The water surface conditions at the landing site were reported as smooth. Because of recent heavy rainfall, the lake level was higher, the water clarity was diminished due to mud run-off, and the lake contained more floating and partially submerged debris than normal. The pilot stated he made three low passes over the landing area prior to his water landing. Examination of the submerged wreckage by salvage divers revealed the nose and left main gear fully retracted and totally in their respective wheel wells, while the right main gear was not fully retracted. The landing gear control handle was in the up position. Subsequent examination of the salvaged amphibian by FAA personnel revealed the hull impacted an unknown submerged object of such mass and weight to crush and tear the left front hull from the nose cone to the cockpit. Examination of the landing gear hydraulic actuators, main gear uplocks, and right main gear door damage revealed the probable sequence of events to be impact with a submerged object early in the touchdown followed by a prying action by the water to partially extend the right main landing gear during the postimpact gyrations. (See enclosed FAA Inspector's statement)