The pilot reported that upon arriving at his intended destination he noted that the winds were light and variable but appeared to favor landing toward the lake on runway 16. Trees were located near the approach end of the 1,765-foot-long turf runway, and the pilot stated he landed "longer than usual." Despite application of brakes, the pilot stated that he was unable to stop before reaching the end of the runway. The airplane went down an embankment and into the lake that is adjacent to the runway's departure end. The airplane came to rest partially submerged. When the airplane was subsequently extracted from the water the firewall was found bent. The pilot made no allegation of having experienced any mechanical malfunction with his airplane during the accident flight.