A HIGHLY EXPERIENCED PLT; WITH VERY LOW TIME IN A C182 BLUNDERED INTO SEA CLASS B AIRSPACE WHILE SIGHTSEEING WITH A PAX.

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RETURNING TO MY HOME FIELD (HARVEY); I WAS SHOWING THE SEATTLE AREA TO MY PAX. IT WAS A BEAUTIFUL DAY AND I HAD IN MY MIND THAT THE FLOOR OF THE CLASS B AIRSPACE WAS AT 5000 FT ON MY PLANNED COURSE RETURNING TO HARVEY. I WAS APPROX OVER THE WINSLOW FERRY DOCK WHEN I AGAIN CHKED MY TERMINAL AREA CHART. I REALIZED THE FLOOR OF THE CLASS B AIRSPACE WAS ONLY 3000 FT MSL. I IMMEDIATELY DSNDED TO BELOW 3000 FT. I FLY ALMOST DAILY OUT OF SEATAC FOR AN AIRLINE. I HAVE JUST RECENTLY RETURNED TO VFR 'SMALL AIRPLANE' FLYING. BEING THAT MY PART 121 FLYING IS ALL IFR; DEALING WITH THE CLASS B/D AIRSPACE IS SOMEWHAT OF A NEW EXPERIENCE IN THE VFR ENVIRONMENT. WHAT CAUSED THIS PROB (AND I AM NOT POSITIVE I DID PENETRATE THE CLASS B; BUT IF NOT I SURE CAME CLOSE -- TOO CLOSE!) WAS NOT DEVOTING 100% ATTN TO MY LOCATION AND RECHKING -- AGAIN AND AGAIN -- MY PLANNED ALT FOR GOING UNDER THE CLASS B. IT WAS A SIT WHERE I HAD MADE A PLAN TO BE AT 3500 FT; THEN I CHKED THE CLASS B FLOOR AND CONFIRMED IT WAS 5000 FT. THEN I DID NOT REALIZE MY ACTUAL COURSE WAS FARTHER S AND ONE THAT WOULD BE IN THE AREA WHERE THE FLOOR WAS ONLY 3000 FT.