A LNDG B767-300 FLC COMPLAINS OF A LATE WARNING FOR DETERMINING THE DECISION HT AUTH FOR AN ILS TO BOS RWY 4R WHEN TWR RPTS 'CAUTION; SHIP IN THE CHANNEL WITH A HT OF 150 FT.' THE AUTH DECISION HT WITH A 'TALL SHIP' IN THE CHANNEL IS 359 FT VERSUS 218 FT NORMALLY. ONE RPTR SUGGESTED THAT THE APCH CTLR GIVE THE ADVISORY.

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AFTER A HOLDING PERIOD OF ABOUT 30 MINS WE BEGAN OUR APCH TO BOS ARPT. WE RECEIVED THE ATIS: RVR TO 6000 FT MID 2000 FT ROLLOUT 4000 FT. WE SWITCHED FREQ FROM APCH TO TWR AT THE MARKER. WHILE INSIDE THE FAF; TWR RPTED 'SHIP IN HARBOR AREA.' THE CAPT WAS FLYING AND THE FO HAD TO CHK THE NEW DECISION HT AND REQUIRED VISIBILITY. AN RVR OF 6000 FT OR 1 1/4 MI WAS REQUIRED AND THAT WAS THE RPTED RVR. WE HAD THE RWY ENVIRONMENT IN SIGHT AT THE NEW DECISION HT OF 359 FT. AN UNEVENTFUL LNDG WAS MADE. THE PROB WAS THE CONFUSION CAUSED BY THE LATE INFO GIVEN BY TWR CTLR INSIDE THE FAF; WHICH IS A HIGH WORKLOAD AREA. I REALIZE SOMEONE HAS TO BE THE FIRST TO GET THIS INFO BUT IT SHOULD BE SUPPLIED BY APCH AND NOT BY TWR. YOU DON'T NEED TO BE WONDERING IF YOU ARE LEGAL WHILE YOU ARE LOOKING FOR THE APCH LIGHTS. SUPPLEMENTAL INFO FROM ACN 405853: SEVERAL HUNDRED FT ABOVE MINIMUMS (AND PAST THE FAF) TWR RPTED A SHIP APCHING AREA R TO L (WITH A 160 FT MAST). ON SHORT FINAL WE CHANGED OUR MINIMUMS FROM 218 FT MSL TO 359 FT MSL (TALL VESSEL IN APCH AREA) MINIMUMS. THE RWY LIGHTS WERE OBSERVED AS WE APCHED OUR NEW MINIMUMS. ON SHORT FINAL WE HAD A BOAT IN SIGHT ALONG WITH THE WATERWAY AND RWY LIGHTS. AT THAT TIME I ASKED TWR IF WE WERE STILL CLRED TO LAND AND THEY SAID YES.