The airplane was landing on runway 27 when it started to veer to the right. The certified flight instructor attempted to take control of the airplane and correct the drift; however, the student did not relinquish the controls and the airplane continued off of the runway. The airplane went into a ditch, nosed over, and came to rest in three feet of water, resulting in substantial damage to the left wing. No preaccident mechanical malfunctions or failures were found that would have precluded normal operation.