THE PRIVATE PILOT WAS DIVERTING TO ANOTHER AIRPORT DUE TO THUNDERSHOWERS AT HIS PLANNED DESTINATION. WHILE ON SHORT FINAL, HE REPORTED HEAVY RAIN CONDITIONS. AFTER TOUCHDOWN, HE REPORTED FORWARD VISIBILITY TO BE ZERO AND THAT THE AIRPLANE WAS HYDROPLANING. CROSSWIND CONDITIONS WERE ALSO REPORTED AT THE TIME OF THE ACCIDENT. HE WAS NOT ABLE TO STOP THE AIRPLANE IN THE REMAINING RUNWAY, AND THE AIRPLANE COLLIDED WITH A DITCH. THREE WITNESSES AT THE AIRPORT REPORTED THAT THE PILOT LANDED HALFWAY OR FURTHER DOWN THE 4,000-FOOT LONG RUNWAY AT A FASTER-THAN-NORMAL SPEED.