PLT DIVERTS TO LAND DUE TO ICING. SLIDES OFF END OF RWY DUE RWY ICING.

Narrative

DURING AN IFR FLT FROM SKI TO SD8; I FLEW INTO ICING CONDITIONS AT 9000 MSL APPROX 25 NM W OF POLO VOR. WE REQUESTED AND DSNDED TO 7000 MSL AND THEN 5000 MSL. WE HAD ACCUMULATED APPROX 3/8 INCHES OF RIME ICE ON THE LEADING EDGES OF THE ACFT AT 8000 MSL AND THEN THE FRONT WINDSHIELD BEGAN TO ICE OVER UNTIL WE (I HAD ONE PAX WHO IS A PLT) COULD NOT SEE THROUGH THE FRONT WINDSHIELD. WHEN THE SHIELD BEGAN ICING; WE ASKED ROCKFORD APCH FOR VECTORS FOR VOR-A AT C73. WE COMPLETED THE APCH AND CAME OUT OF THE CLOUDS AT 1600 MSL. I SAW THE ARPT FROM THE SIDE WINDOW AND HAD PLANNED TO LAND ON RWY 30. THE WINDS WERE RPTED BY ROCKFORD APCH AT 30-36 AND 10 KTS. AT THE SAME TIME MY PAX SAW A WATER TWR FROM THE OTHER SIDE OF THE ACFT. THE WATER TWR IS NOT IDENTED ON THE APCH PLATE SO IT WAS A SURPRISE TO US. I COULD NOT MAKE MY INTENDED APCH WITHOUT LOSING SIGHT OF THE WATER TWR AND OR THE ARPT SO I DECIDED TO LAND ON RWY 12 BY MAKING A 30 DEG L TURN. UPON TOUCHING DOWN. THE BRAKING WAS NIL AND WE SKIDDED APPROX 1800 FT DOWN THE RWY 12 OFF THE END OF THE RWY AND THROUGH A BARBED WIRE FENCE. NO ONE ANSWERED ON UNICOM SO WE WERE UNABLE TO GET ANY ACTUAL ARPT INFO PRIOR TO LNDG OTHER THAN WHAT WAS PROVIDED BY OUR APCH PLATE. I BELIEVE THE CONTRIBUTING FACTORS TO THIS INCIDENT WERE FLT INTO ICING CONDITIONS; UNMARKED OBSTACLES NEAR THE ARPTS AND ICING ON THE RWYS; AND OUR SELECTION OF RWYS WHICH WAS BASED ON REDUCED VISIBILITY AND OUR BELIEF THAT CONTINUED FLT IN THE ICE COULD BE TOTAL.