According to the pilot, the aircraft was on its first flight after a major engine overhaul and was being flown for maintenance evaluation. About 45 minutes after takeoff, the pilot requested a descent from 7,500 feet MSL. After entering an overcast, the engine sputtered and began to vibrate and lose power. The engine then stopped and would not restart, and the pilot declared a "May Day." The plane broke out of the overcast over a lake at about 2,200 feet. Subsequently, the pilot made a controlled landing near boats about one mile from the shore. The occupants were rescued in less than a minute. Later, the aircraft was recovered from the lake where it had sunk in about 90 ft of water. An engine teardown revealed that the No. 5 connecting rod had separated from the piston. The stub of the connecting rod had beat a hole in the left side of the crankcase, which in turn, resulted in oil starvation.