On August 12, 1998, approximately 1300 central daylight time, a Cessna 185F amphibian airplane, N4810, was substantially damaged following a loss of control during high speed taxi after landing at Brenton Sound near Venice, Louisiana. The commercial pilot and his passenger were not injured. Visual meteorological conditions prevailed and a company flight plan was filed for the Title 14 CFR Part 91 business flight which originated from Lacombe, Louisiana, at 1230. The airplane was owned and operated by Southern Natural Gas Company of Birmingham, Alabama. According to a written statement provided by the pilot, he noticed a boat departing the landing area and determined that the boat would be clear prior to landing. After landing, the pilot started a "high speed step-taxi." The pilot noticed a swell left by the boat and decided to approach it at a 45 degree angle. As the airplane crossed the swell the pilot felt the right float "dig in." The pilot added full power and pulled full aft on the yoke in an effort to prevent the floats from going under water any farther. Subsequently, the aircraft nosed over inverted and began to sink. The pilot reported that he opened his door, unbuckled his safety belt and exited the airplane. As he surfaced, he noticed that his passenger had not followed, so he elected to return to the airplane and assist his passenger. At the time of the accident, the pilot had accumulated a total of 16,076 flight hours, of which 9,085 hours were in the same make and model as the accident aircraft. FAA inspectors initially reported this occurrence in August and the damage was classified as minor. The aircraft was inspected again in November by a FAA inspector and the damage was re-classified as substantial. The FAA inspector stated that the engine mount was broken in half and the fire wall was buckled. The NTSB South Central Region was notified of this occurrence on November 17, 1998.