B737-500 DESCENDED WELL BELOW GLIDE PATH ON VISUAL APCH. DEVIATION RECOGNIZED AT 1000 FT; WHEN THE FLT CREW LEVELED TO RECAPTURE NORMAL GLIDE PATH.

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ON APCH TO BOS RWY 27; ATC (APCH CTL) TURNED US ON A 90 DEG BASE THAT WOULD HAVE PUT US ON 10+ MI FINAL. THEN APCH CTL TURNED US (ALMOST) DIRECTLY TO THE FAF AND CLRED US FOR A VISUAL APCH. THE CAPT WAS PROBABLY HIGH/FAST AND DEPLOYED FULL SPD BRAKES. WHILE LOOKING OUTSIDE AND SWITCHING FROM APCH TO TWR; THE CAPT DSNDED THE AIRPLANE WELL BELOW THE FAF ALT OF 1700 FT. AT 1000 FT I NOTICED THE PAPI'S WERE ALL RED AND LOOKED DOWN AND SAW THE ALTIMETER OF 1000 FT. AFTER CHKING IN WITH TWR; TWR MADE THE STATEMENT; 'ACR X; I GOT AN ALT ALERT. YOU OK?' I REPLIED 'WE ARE OK.' THE CAPT LEVELED THE AIRPLANE OFF AT 1000 FT AND CAPTURED THE GS/PAPI FROM THERE. THIS (OF COURSE) IS OUT OVER THE WATER; VERY LITTLE (IF ANY) GND REF IN THE APCH AREA. I DO NOT KNOW WHAT ALT WAS SET IN THE ALT WINDOW AND I DO NOT HAVE IF IT ALERTED US TO ANY ALT. UPON CLRNC FOR A VISUAL APCH AND TURNED TOWARD THE FAF; THE CAPT OR I SHOULD HAVE SET 1700 FT IN THE ALT ALERT WINDOW AND TARGETED THAT ALT AS THE MDA UNTIL ESTABLISHED ON THE GLIDE PATH. DSCNT TO 1000 FT AT RIPIT WAS NOT ACCEPTABLE AND I FEEL THIS MISTAKE IN MORE RUGGED TERRAIN WOULD BE BAD. REAL BAD. AS MONITORING PLT; I FAILED TO MONITOR THIS LOW ALT SITUATION.