At about 10:37 eastern time, a Cessna 210L lost power shortly after takeoff and was ditched near Key West. The pilot, and three passengers were not injured. One passenger sustained minor injuries. The pilot said she was level at 700 feet agl and reduced power to 25 inches and 2500 rpm when she heard a loud bang. The rpm and oil pressure gauges indicated zero but the propeller continued to rotate. The doors were opened and the passengers were advised to brace before ditching. After the ditching, she got her daughter who was one of the passengers out of the airplane and she believes that the other adult passenger assisted the other two passengers out of the airplane. Preliminary examination of the engine revealed that the crankshaft was fractured just aft of the No. 2 main bearing; main bearing pieces were found in the oil sump. Additionally, one of the bolts of the No. 2 cylinder connecting rod was fractured near the bolt head.