A LNDG DH8 GOES BELOW 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.

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ENRTE WE BRIEFED FOR THE ILS RWY 4R APCH BASED ON ATIS AND APCH CTL INFO THAT RVR WAS 4000 FT. WE WERE TRACKING INBOUND ON THE LOC AND APCH KEPT US HIGH (ABOVE GS) BEFORE CLRNC FOR THE APCH. I ELECTED TO FLY THE APCH MANUALLY TO FACILITATE INTERCEPTING THE GS FROM ABOVE (I WAS UTILIZING THE FLT DIRECTOR). WE CONTACTED THE TWR AT MILTT PER INSTRUCTIONS FROM APCH. NOT LONG AFTER MILTT; I HEARD THE TWR ISSUE A 'CAUTION' TO THE ACFT AHEAD OF US THAT THERE WAS A SHIP IN THE CHANNEL WITH A HT OF 150 FT. THE ACFT AHEAD SUBSEQUENTLY LANDED AND WAS CLRED OFF THE RWY. THE TWR CTLR THEN ISSUED THE SAME 'CAUTION; SHIP IN CHANNEL; 150 FT IN HT' TO US. AT THIS POINT; WE WERE OVER 1/2 WAY BTWN MILTT AND THE RWY. WHILE CONCENTRATING ON FLYING THE APCH; IN THE 'BACK OF MY MIND' I WAS TRYING TO CONSIDER THE SIGNIFICANCE OF THE CAUTION. WE CONTINUED THE APCH AND MADE CONTACT WITH THE APCH LIGHTS JUST ABOVE THE NORMAL DECISION ALT (218 FT). AFTER LNDG AND TAXIING IN; WE LOOKED OVER THE APCH CHART AND REALIZED THE 'CONDITIONAL DECISION ALT' FOR TALL VESSELS MAY HAVE APPLIED. IN FACT; I DID NOT KNOW WHAT HT CONSTITUTES A 'TALL VESSEL.' IT IS NOT WRITTEN ANYWHERE THAT I COULD FIND. I ASKED CLRNC DELIVERY AND THEY DID NOT KNOW; BUT THEY CHKED AND TOLD US IT WAS 85 FT OR HIGHER (OOPS!). WE WERE CLRLY REMISS IN NOT CATCHING THE 'CONDITIONAL' DECISION ALT DURING THE BRIEFING; BUT THERE WERE SEVERAL ISSUES THAT 'SET THE TRAP' FOR US. FIRST; THERE WAS NO MENTION OF SHIPS IN THE CHANNEL UNTIL WE WERE WELL INSIDE MILTT. (IF WE HAD KNOWN ABOUT THE SHIPS PRIOR TO GS INTERCEPT WE MIGHT HAVE NOTICED THAT WE DID NOT HAVE MINIMUMS (RVR 6000 FT) FOR THE APCH.) SECOND; THE CTLR DID NOT USE THE TERMINOLOGY 'TALL VESSELS;' WHICH GAVE US AN AMBIGUOUS CAUTION MESSAGE. A MORE CLR STATEMENT WOULD BE 'TALL VESSELS IN APCH AREA -- HIGHER MINIMUMS APPLY.'