THE AIRPLANE RAN OFF THE END OF THE RUNWAY DURING A REJECTED TAKEOFF, COMING TO REST ON A DIKE AND A TIDAL MUD FLAT. FDR DATA CONFIRMED THAT THE PITOT HEAT HAD NOT BEEN SELECTED 'ON' BY THE FLIGHTCREW. A BUILDUP OF SNOW AND/OR ICE IN THE PITOT/STATIC SYSTEM TUBES AND PORTS RESULTED IN ERRONEOUS AIRSPEED READINGS DURING THE ATTEMPTED TAKEOFF. SUBSTANTIAL DEVIATIONS FROM CHECKLIST PROCEDURES WERE RECORDED ON THE CVR. 19 SECONDS ELAPSED BETWEEN THE TIME THAT THE AIRPLANE ACTUALLY ACCELERATED THROUGH 60 KIAS (THE FIRST MARK ON THE AIRSPEED INDICATOR) AND THE START OF THE REJECTED TAKEOFF.