On January 31, 2001, at 1040 eastern standard time, a Cessna 208, N208KW, collided with water during landing off-shore in the Dry Tortugas National Park near Fort Jefferson, 70 miles west of Key West, Florida. The airplane was operated by Seaplanes of Key West, and flown by the Airline Transport Pilot (ATP) under the provisions of Title 14 CFR Part 135, and visual flight rules. Visual meteorological conditions prevailed and a visual flight rules flight plan was filed for the local sightseeing flight. The pilot and nine passengers were not injured and the airplane sustained substantial damage. The flight originated from Key West, Florida, at 1000. According to the pilot, while making a normal approach, including a flyby to evaluate landing conditions, he observed that one of the ferryboats was arriving in the south channel. He planned and performed a water landing to avoid any past wake swells created by the ferry boat in the landing area. Approximately 1/2 to 3/4 through the landing slide, with the airplane slowed to approximate 30 - 35 knots, a large swell appeared and launched the airplane 10-15 feet above the water. This swell was completely separated from the boat wake that had been observed earlier. The pilot further stated that, at this point, he was "powerless" to do anything but wait for the airplane to settle. After the airplane settled on the water, the pilot taxied to shallow water where assistance was received from park service in deplaning the airplane. The examination of the airplane disclosed that the rear float/strut attach point had failed and the strut assembly had penetrated the airframe skin. No mechanical problems with the airplane were reported by the pilot. According to the pilot, the swell that launched the airplane during the landing was not seen until it was too late for him to avoid the collision.