The pilot reported that the take-off on runway 17 proceeded normally until immediately after the gear was selected up. At this point the aircraft pitched up and rolled to the left. The pilot pushed the auto-pilot release button on the control column and heard the release annunciator sound, however he noted that the yaw damper and auto-pilot annunciators were illuminated. He also noted the elevator trim wheel moving and shortly thereafter the stick shaker began to operate at an aircraft speed of about 115 knots. The aircraft was manoeuvred for a successful landing on runway 26. It was determined that water had entered the auto-pilot computer via a faulty seal on the electronics compartment door. Prior to take-off the pilot armed the flight director system in the indicated air speed and heading modes. During the take-off run the auto-pilot engaged, wound the elevator trim nose up and commanded a left turn. The pilot managed to overpower the auto-pilot inputs by applying extremely high forces to the control column, but in doing so was unable to withdraw one hand from this task to trip the circuit breaker controlling power to the auto-pilot.