PLT OF A DEHAVILLAND OTTER DHC3 TURBOPROP FLEW CLOSER THAN 500 FT TO PERSONS AND VEHICLES WHEN AVOIDING LOW CEILINGS OVER WATER DURING AN AIR TAXI FLT.

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I AM A SENIOR; FULL-TIME PLT FOR; ATX; A PART 135 SEAPLANE OPERATOR BASED ON THE LOCATION OF ABC LAKE NEAR SEATTLE. AFTER SEVERAL STOPS IN THE SAN JUAN ISLANDS ON A SCHEDULED FLT; I DEPARTED ROCHE HARBOR ON THE RETURN LEG WITH 6 PAX. RAIN; MIST AND FOG IN THE ISLANDS AND THE STRAIT OF JUAN DE FUCA HAD HAMPERED THE FLT; SO I DECIDED TO RETURN ON A MORE WESTERLY RTE THAN NORMAL. FELLOW COMPANY PLTS ON OUR VICTORIA; BC; RUNS WERE RPTING BETTER WX IN THAT AREA. PROCEEDING SOUTHWESTWARD ALONG THE OUTSIDE COAST OF SAN JUAN ISLAND; I CRUISED AT APPROX 400 FT. VISIBILITY APPEARED TO BE ABOUT 3 MI IN MODERATE RAIN. THE SKY SEEMED TO BE LIGHTENING AND THE CEILING RISING; DEVELOPMENTS CONSISTENT WITH THE RPTS FROM THE VICTORIA PLTS. I HAD NO REASON TO EXPECT ADVERSE CONDITIONS IMMEDIATELY AHEAD. BUT SUDDENLY THE AIRPLANE ENCOUNTERED SOLID CLOUDS AT OUR ALT. THEY HAD BEEN UNDETECTABLE FROM THE RAIN AND SKY; BEING OF THE SAME SHADE OF GRAY. TO AVOID GOING INTO IMC; I HAD TO MAKE AN ABRUPT DSCNT TO ABOUT 200 FT. VISIBILITY LOOKED MUCH WORSE AHEAD; AND I WAS LOATHE TO LAND DUE TO THE SWELLS AND EXPOSED NATURE OF THE STRAIT OF JUAN DE FUCA. INSTEAD; SINCE THE SHORELINE WAS STILL IN SIGHT; I CHOSE TO EXECUTE A 180 DEG TURN TOWARDS THE LAND. WHILE IN THE TURN; I NOTICED SEVERAL SMALL BOATS. ALTHOUGH CONTACT WITH THEM WAS AT NO TIME A POSSIBILITY; I MAY HAVE MOMENTARILY BEEN SLIGHTLY CLOSER TO THEM THAN THE 500 FT ALLOWED BY LOW. HOWEVER; I BELIEVE I CHOSE THE SAFEST OPTION AVAILABLE. THE REST OF THE FLT WAS UNEVENTFUL. WE RETURNED TO OUR TERMINAL VIA THE OUTBOUND RTE IN LOW BUT LEGAL AND SAFE CONDITIONS. I'M FILING THIS RPT BECAUSE I'M CONCERNED THAT THE OCCUPANTS OF THE BOATS MIGHT HAVE THOUGHT WE WERE TOO CLOSE TO THEIR VESSELS AND MIGHT HAVE ASSUMED WE WERE CRUISING AT LOW ALT IN POOR VISIBILITY. I WANT TO EXPLAIN THAT WE WERE IN THOSE CONDITIONS ONLY MOMENTARILY WHILE TURNING TO ESCAPE AN UNEXPECTED AND UNUSUALLY SHARP DETERIORATION OF WX. OTHER COMPANY PLTS THAT DAY WERE ALSO EXPERIENCING PROBS WITH THE WX. IN FACT; 6 OF OUR ACFT; INCLUDING THOSE FLOWN BY THE CHIEF PLT AND THE DIRECTOR OF OPS; WERE UNABLE TO RETURN HOME AND SPENT THE NIGHT IN OUTLYING HARBORS AS CONDITIONS CONTINUED TO WORSEN THROUGHOUT THE DAY.