APCH TO WRONG ARPT. BOEING FIELD IS VERY CLOSE TO SEATAC ARPT AND RPTR MISTOOK SEA FOR BFI. ATC SAW ACFT ON APCH TO SEA AND VECTORED RPTR TO BFI. APCH CTLR TOLD RPTR THE MISTAKE HAPPENS REGULARLY.

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APCHING FROM THE S OF SEA ON THE 70 DEG RADIAL OF OLM; TALKING WITH SEA APCH. CONDITIONS: SMOOTH; CLR; VISIBILITY UNLIMITED. ENRTE TO AND LNDG BFI. ATIS SAID RWY 31L LOC BACK COURSE IN USE AT BFI. I HAD BOTH NAV #1 AND NAV #2 TUNED TO APPROPRIATE LOC FREQ; AS BACKUP FOR VISUAL APCH. CTLR HAD CLRED ME AT ABOUT 10 DME ON MY GPS TO THE VISUAL RWY 31L AT BFI (AFTER I HAD RPTED THE FIELD IN SIGHT -- WHICH I FOUND OUT LATER WAS SEA; AND IS THE REASON FOR THIS RPT). APCH FURTHER INSTRUCTED ME NOT TO DSND BELOW 3000 FT UNTIL 'TURNING FINAL.' MY LAST LOOK AT MY NAV INSTS SHOWED 8.5 DME AND 1 DOT FROM CTRING CDI ON MY HSI. I WENT VISUAL AFTER THAT. AS I TURNED FINAL; I STARTED MY DSCNT OUT OF 3000 FT. AFTER A SHORT WHILE 2 THINGS CAUGHT MY ATTN: 1) THERE WAS A LARGE TERMINAL I HADN'T EXPECTED TO SEE; AND 2) BOTH RWYS WERE RELATIVELY LONG AND I HAD EXPECTED TO SEE A SHORT R RWY (FROM MY PREFLT LOOK AT THE RWYS TO SEE IF THE SHORT ONE WOULD BE ACCEPTABLE). I THEN QUESTIONED APCH (I SAID SOMETHING TO THE EFFECT: 'WHICH RWY DID YOU WANT ME ON?') AT THAT POINT I WAS 2400 FT. ATC REALIZED I WAS LINED UPON SEA; AND GAVE ME A NEW HDG AND TO MAINTAIN AT/ABOVE 2000 FT. I STOPPED MY DSCNT AT 2200 FT. I ASKED ATC IF THAT WAS THE FIRST TIME THAT HAD HAPPENED AND HIS REPLY WAS: 'NOPE; AND IT WON'T BE THE LAST.' I CONTINUED AND LANDED SAFELY AT BFI. CONTRIBUTING: MY UNFAMILIARITY WITH THE AREA; RELATIVE CLOSENESS; SIZE; AND ALIGNMENT OF RWYS AT SEA AND BFI; MY THINKING AT 8.5 MI THAT THE ARPT WAS SO CLOSE (I HAD DECIDED THE SEVERE CLR MADE THINGS FURTHER THAN APPEARED); LACK OF A CAUTION FROM APCH ON SIMILARITY AND CLOSENESS OF THE 2 ARPTS; LACK OF ATC QUESTIONING MY DSCNT SO SOON OUT OF 3000 FT; AND I COULDN'T SEE BFI FROM MY POS; BUT SEA WAS VERY OBVIOUS. GIVEN APCH'S COMMENT THAT THIS WOULDN'T BE THE FIRST OR LAST TIME THIS WOULD HAPPEN; AND SINCE I COULD HAVE EASILY BEEN DSNDING ON A PLANE GOING TO SEA; I THINK SOME BRIEF CAUTION FROM APCH ON DOUBLE-CHKING THE IDENT OF SEA VERSUS BFI WOULD BE BENEFICIAL.