AN ATR72 DISPATCHED WITH THE CABIN PRESSURIZATION SYS INOP AND DEFERRED DID NOT COMPLY WITH THE OVERWATER RESTRS.

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ACFT WAS DISPATCHED WITH AUTO AND MANUAL PRESSURIZATION INOP. MEL REQUIRED ACFT TO BE OPERATED IN AN APPROVED UNPRESSURIZED CONFIGN. OUR FLT MANUAL GIVES THE SPECIFICS ON THIS PROC. THE MOST IMPORTANT BEING THE OPENING OF BOTH OUTFLOW VALVES BY MAINT. THIS WAS DONE; AND PROPERLY SIGNED OFF BY MAINT PERSONNEL. THE FLC IS ALSO SUPPOSED TO CONFIGURE DIFFERENT VALVES IN THE COCKPIT. THIS WAS ALSO DONE CORRECTLY. LASTLY; THE MEL PROHIBITS FLT ABOVE 10000 FT MSL IN THIS CONFIGN; AS WELL AS OVERWATER FLT BEYOND THE GLIDE DISTANCE OF THE ALT BEING FLOWN. WE PICKED UP THE ACFT FOR THE EARLY MORNING KICK-OFF FLT. WE SAW THAT WE WOULD COMPLY WITH EVERY MEL PROVISO; SO OFF WE WENT. ALSO; WE SAW THAT THE PAPERWORK INDICATED THAT WE WOULD SWAP ACFT FOR THE NEXT LEG; WHICH WOULD TAKE US OVER LAKE MICHIGAN. ARRIVING IN THE HUB; WE FOUND THAT THE SWAP WOULD NOT OCCUR; AS WE THOUGHT. WE WERE A LITTLE BEHIND; AND HAD A SHORT TURN AROUND TIME; WHICH MEANT WE WERE HUSTLING TO GET THE FLT OUT. HAVING CHKED THE MEL EARLIER; WE DIDN'T CHK IT AGAIN; COMPLETELY FORGETTING THE OVERWATER PROVISO. WE TOOK THE ACFT OVERWATER NOT REALIZING THIS; UNTIL; ON OUR RETURN LEG; A DISPATCH NOTE INFORMED US OF OUR COMPANY'S PMI; PRIMARY MAINT INSPECTOR; DECISION TO WAIVE THE OVERWATER RULE. WE DECIDED THAT WE HAD IN FACT BEEN TECHNICALLY 'LEGAL' TO MAKE THE FLT; BUT ALSO TECHNICALLY 'COMPLACENT' BY NOT FINDING THE INCONSISTENCY SOONER; AND CERTAINLY; BEFORE THE FLT TOOK OFF.