At the beginning of the flight, during the takeoff ground run, the pilot realized that he had not removed the airplane's control gust lock. The pilot was unable to stop the airplane before it travelled off the end of the runway, collided with a fence and came to rest partially submerged in the bay north of the airstrip. The pilot reported that the airplane was normally kept in a hangar and that the gust lock was not used. The day before the accident, another pilot had flown the airplane and left it outside the hangar, and had installed the gust lock. The lock that was used was just the pin, as the red caution flag had previously broken off. The pilot did not see that the pin was installed during the preflight inspection. The pilot also reported that he did not use a checklist and at no time during the preflight inspection did he check for freedom of movement of the flight controls.