B757 CAPT FAILS TO MEET RECOMMENDED ALTS ON RIVER VISUAL APCH TO DCA.

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WE WERE DELAYED OVER 1 HR PAST SCHEDULED DEP BEFORE FINALLY 'PUSHING BACK' ON OUR DTW-DCA LEG. I SHOULD ALSO NOTE THAT THE FO WAS NEW TO THE B757; HAVING RECENTLY COMPLETED INITIAL TRAINING. STANDARD UNEVENTFUL FLT TO DCA. THE ATIS BROADCAST STATED THE ILS TO RWY 1 WAS CURRENTLY BEING UTILIZED. DURING OUR DSCNT AND HDOF TO POTOMAC APCH; WE LEARNED THAT THE APCH HAD CHANGED TO THE RIVER VISUAL TO RWY 19. THE FO HAD NEVER FLOWN THIS APCH ON THE B757; HENCE; I BOTH 'LOADED' THE DATA INTO THE FMC AND INSTRUCTED HIM AS TO HOW WE WOULD FLY THE APCH. DCA WX WAS VFR. ON A FINAL VECTOR TO THE POTOMAC RIVER; THE FMC 'DUMPED' THE DATA. I VISUALLY ACQUIRED THE RIVER; DISCONNECTED THE AUTOPLT; AND REFED 'RAW DATA' TO AID OUR APCH. PLUS; OF COURSE; FOLLOWING THE RIVER. I SET UP 1000 FPM DSCNT RATE ON VERT SPD AND 'HDG' ON THE HDG BUG WHILE SLOWING TO FINAL APCH SPD. THE CHARTED 'RECOMMENDED' ALTS ARE 1200 FT AT 4 MI; AND 900 FT AT 3 MI. WE DSNDED TO BTWN 750-800 FT AT 4 MI WHEREBY I QUICKLY LEVELED OFF. WE WERE PROPERLY OVER THE RIVER BUT LOWER THAN I WANTED TO OR SHOULD HAVE BEEN. MY FAULT AS I BECAME DISTR BY THE FO'S QUESTIONS DURING THE APCH PLUS I WAS BASICALLY FLYING 'ALONE' HENCE SHOULD HAVE MAINTAINED THE AUTOPLT 'ENGAGED' INSTEAD OF HAND FLYING THE JET. I MAINTAINED 800 FT UNTIL THE APPROPRIATE POS TO CONTINUE DSCNT AND LANDED UNEVENTFULLY. NO VIOLATION BUT BAD JUDGEMENT ON MY PART MANAGING THE FLT DECK.