The pilot reported that it had just rained on the 2,695-foot-long runway that he intended to land on. During the landing roll, the pilot applied moderate braking but he did not sense that the airplane was decelerating sufficiently. The pilot stated that his knowledge of this airport made him aware of a 150 foot drop off located at the end of the runway. With the end of the runway approaching, the pilot elected to turn left off the runway and he used the right engine to assist the turn. The airplane veered left off the end of the runway onto a gravel area. When the airplane entered the gravel area the right main landing gear collapsed, resulting in substantial damage to the outboard edge of the aileron, and damage to the landing gear and top of the inboard wing. The pilot said there were no preimpact mechanical malfunctions or failures.