DHC8 FLT CREW HAS A RWY EXCURSION DURING LNDG AT SEA.

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ON THE CHINS5 ARR TO SEA; ATIS AND ATC WERE CONTINUOUSLY RPTING RVR'S RANGING FROM 1000-1600 FT RVR. ONCE THE ACFT WAS ON THE DOWNWIND LEG FOR THE ARR FOR THE RWY 16C ILS CAT 3 WE RECEIVED A RPT THAT THE RVR WAS DOWN TO 500 FT. CONSIDERATIONS BEGAN AS TO OTHER OPTIONS AS WE WERE GIVEN A DSCNT TO 7000 FT. BY THE TIME THE ACFT WAS ON THE BASE VECTOR; THE WX WAS UPDATED TO 600 FT RVR; THE MINIMUM FOR OUR ACFT. THE ACFT WAS SLOWED TO APPROX 200 KTS PER PROFILE. ON FINAL; ATC REQUESTED A SPD OF 180 KTS AND WE WERE CLRED FOR THE APCH. THE ACFT WAS CONFIGURED AND SLOWED PER THE STANDARD PROFILE BY THE FAF AT 6.1 DME. AT APPROX 5 DME WE RECEIVED ANOTHER UPDATE OF 500 FT RVR; PER OUR OPS SPECS WE WERE ABLE TO CONTINUE IF THE SUB-MINIMUM RPT IS MADE INSIDE THE FAF; HOWEVER AT ABOUT 500 FT AFE WE RECEIVED ANOTHER RPT OF 600 FT RVR. THE ACFT WENT IMC AT ABOUT 200 FT (THE TWR CAB WAS VISIBLE ABOVE THE CLOUD DECK). JUST PAST THE 100 FT TO GO CALL I CALLED THE RWY IN SIGHT; WITH THE REQUIRED RWY ENVIRONMENT AVAILABLE; HOWEVER I DID NOTICE THE ACFT LINED UP L OF CTRLINE DESPITE BEING IN THE HGS GUIDANCE CUE. THE ACFT WAS FLOWN ONTO CTRLINE AND LANDED; INSIDE THE TOUCHDOWN ZONE. NORMAL SLOWING PROCS WERE USED. A COMMENT WAS MADE ABOUT THE HGS GUIDANCE FROM CTRLINE AND THEN WE BEGAN TO TALK ABOUT THE SMGS REQUIREMENT FOR AN EXIT AT TXWY P; WHICH IS AT THE FAR END OF THE ACTIVE RWY. WE VERIFIED THIS EXPECTATION; WITH ATC; SINCE THE RVR HAD BEEN VARIABLE. WE RECEIVED NO CLARIFICATION; ONLY SOMETHING TO THE EFFECT OF 'EXIT L.' THE VISIBILITY MADE IDENTING THE TXWYS DIFFICULT; AND ONCE I FELT WE WERE NEAR OUR TXWY; IT WAS EVIDENT FROM PAST EXPERIENCE THE ACFT WAS NOT GOING TO BE ABLE TO TURN. ADDITIONAL BRAKING WAS APPLIED AND QUICKLY BROUGHT TO MAX; SOON AFTER THE ACFT WAS BROUGHT INTO REVERSE. IT WAS QUICKLY APPARENT TO ME THAT THE ACFT WAS NOT SLOWING AS I EXPECTED; NOR DID IT SEEM THAT ANTI-SKID WAS OPERATIONAL. THE BRAKES WERE QUICKLY MODULATED OFF AND BACK ON WITH SIMILAR RESULT. WHEN THE END OF THE RWY CAME INTO VIEW AND ITS ASSOCIATED TURNOFF AT QUEBEC INTXN I HAD SERIOUS DOUBTS THE ACFT COULD COMPLETE THE TURN SAFELY. I FELT THAT THE POSSIBILITY OF THE ACFT SKIDDING; SLIDING; OR BECOMING UNCONTROLLABLE WAS LIKELY. I DECIDED AT THAT MOMENT CONTINUING STRAIGHT AHEAD WAS THE SAFEST OPTION AS THE ACFT WOULD REMAIN CONTROLLABLE; THE OVERRUN WAS ASPHALT AND THE DAMAGE IF ANY WOULD BE MINIMAL. AFTER THE ACFT CAME TO A STOP; THE #2 TIRE (L MAIN; INSIDE TIRE) HAD BLOWN; POSSIBLY AFTER CONTACTING A LIGHT ASSEMBLY. THAT LIGHT CAME FORWARD AND CONTACTED THE PROP ARC; AND 3-4 BLADES. CFR RESPONDED AT OUR REQUEST; I WAS NOT ABSOLUTELY CERTAIN OF THE STATUS OF THE AIRPLANE AND DID NOT WANT FURTHER DAMAGE TO THE ACFT OR LIGHTS. AFTER A FEW MINS THE #1 ENG WAS SHUT DOWN TO ALLOW THE L MAIN ASSEMBLY TO BE SAFELY INSPECTED; BECAUSE THE APU WAS ON MEL I ELECTED TO KEEP THE #2 AT MINIMUM PWR. WE WERE EVENTUALLY TOWED TO THE GATE AND SHUT DOWN BOTH ENGS FOR THAT OP.