FLC OF AN MLG LOST CTL OF THE ACFT DURING LNDG; RESULTING IN GOING OFF THE END OF THE RWY.

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ON A SCHEDULED FLT FROM NEW ORLEANS TO ATL; WE BEGAN OUR ILS APCH AFTER HOLDING FOR APPROX 40 MINS. AT AROUND 10 MI WE ENCOUNTERED A 20 KT LOSS IN AIRSPD FOLLOWED BY A 40 KT GAIN IN AIRSPD. THE FO WAS FLYING THE APCH; AND I SUGGESTED WE CARRY ABOUT 20 EXTRA KTS AND DON'T LET THE PWR GET BELOW 1.2 EPR ON THE APCH. ATIS HAD CALLED THE CEILING AT 2200 FT; BUT WE DID NOT BREAK OUT UNTIL 400 FT AGL. WE WERE IN POS TO LAND (ON AND ON) SO WE CONTINUED THE APCH. TWR CLRED US TO LAND ON SHORT FINAL BUT DID NOT GIVE US THE WINDS. AS THE FO BEGAN HIS FLARE WE WERE ABOUT 25 KTS ABOVE BUG. HE WAS TRYING TO LET THE SPD BLEED OFF BUT DIDN'T WANT TO BRING THE ENGS TO IDLE. IT FELT AS THOUGH WE HAD A TAILWIND DURING THE FLARE. THE ACFT TOUCHED DOWN ABOUT 3000 FT DOWN THE RWY. (RWY 9R IS 9000 FT LONG.) THE FO SELECTED REVERSE THRUST AND BEGAN BRAKING. THERE WAS A GREAT DEAL OF STANDING WATER ON THE RWY AND IT WAS RAINING VERY HARD. THE ACFT DID NOT SEEM TO BE SLOWING DOWN; SO I BEGAN TO APPLY MORE BRAKE PRESSURE AND MORE REVERSE THRUST. I COULD FEEL THE ANTI- SKID SYS CONTINUALLY RELEASE AND REAPPLY THE BRAKES. I INCREASED THE REVERSE THRUST TO THE POINT OF COMPRESSOR STALLS. THE BRAKES WERE OF LITTLE EFFECT AS THE ACFT HYDROPLANED OFF THE END OF THE RWY. WE STOPPED 100 FT OFF THE END. IF I HAD KNOWN OF A TAILWIND I WOULD HAVE EXECUTED A GAR. TWR WINDS WERE RPTED TO BE 300 DEGS AT 26 KTS 5 MINS AFTER OUR LNDG. I SHOULD HAVE REQUESTED A WIND CHK AFTER OUR LNDG CLRNC.