NMAC BETWEEN ACR LGT ON APCH AND GA SMA.

Narrative

WE WERE CLRED THE BAY VISUAL AT SEA (16R). ON GLIDE SLOPE . ABEAM THE NORTH END OF THE RWYS AT BOEING FIELD (JUST INSIDE THE 'PARK' OUTER MARKER); WE PASSED DIRECTLY OVER A TWIN SMA I IMMEDIATELY ASKED TWR IF THEY HAD SEEN THE TWIN ON RADAR. THEY DID NOT REPLY; AND I ASKED AGAIN. TWR SAID THEY DID NOT SEE THE TFC. I WAS SURPRISED. SO I CALLED TWR; ON THE GND AND ASKED WHY THEY DID NOT SEE OUR TFC. THE TWR SUPERVISOR SAID THAT ATC'S RADAR HAD DIFFICULTY PICKING-UP TFC IF THEY WERE LOW-LEVEL AND IN OUR RELATIVE POSITION ON FINAL. I THINK THAT SEA HAS ALL THE INGREDIENTS FOR A MIDAIR COLLISION: INADEQUATE RADAR COVERAGE (INSOFAR AS LOW LEVEL VFR TFC IS CONCERNED). VFR CORRIDOR UNDERLYING THE FLOOR (1600' MSL) OF THE TCA. IN FACT; IT UNDERLIES THE GLIDE SLOPE BY ONLY APPROX 200 VFR TFC DEPARTING NEARBY BOEING FIELD.THE FIELD IS ADJACENT TO THE 16L AND 16R APCH S TO SEA. NO APPARENT COORDINATION BETWEEN TWRS OF EITHER ARPT WHERE BOEING FIELD DEPS CONFLICT WITH ACFT MAKING APCH TO SEAM. RECOMMEND: LOWER TCA TO SURFACE (VICE 1600') ALL ALONG THE 16R ILS FINAL SEQUENCE. GET BETTER RADAR COORDINATION BETWEEN TWR TO PREVENT POTENTIAL CONFLICTS.