After takeoff, the pilot made a normal climb to about 8,500 ft. He then noticed indications of higher than normal EGT and fuel pressure and a lower than normal fuel flow. The pilot reported that he had leveled off and throttled back when the engine began to sputter. He noticed a slight RPM loss with the same fuel pressure and flow readings. He elected to return to the South Lake Tahoe Airport. While returning, there was an additional loss of power and the aircraft was subsequently ditched in Lake Tahoe. The pilot and passenger egressed through a side window and swam to shore. The aircraft was retrieved from the lake. The engine was inspected, and after minor repairs, it operated normally using the aircraft's fuel system. The fuel boost pump could not be tested due to impact damage. The magnetos tested satisfactorily. The spark plugs were too rusty to check; therefore, other plugs were used for the engine run.