B757-200 CREW RETURNED TO THEIR DEP ARPT AFTER THE WATER FILL VALVE WAS NOT IN THE CLOSED POS. THEY DID NOT HAVE POTABLE WATER.

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ACFT SVC ERROR. WE WERE IN CRUISE ON AN OCEAN TRACK APCHING AN INTXN WHICH IS JUST ABOUT THE AREA THAT WE LOSE ACARS COM WHEN THE FLT ATTENDANTS RPTED THAT THEY HAD NO WATER FOR GALLEYS. WE HAD A POTABLE WATER SVCING SLIP AND THE GAUGE IN THE FORWARD GALLEY READ 'FULL.' AS THIS IS A 5 HR FULL FLT; I WAS INTERESTED IN GETTING WATER PRESSURE IF POSSIBLE; SO FINALLY CALLED MAINT (VHF) TO SEE IF THEY COULD SUGGEST ANYTHING. IN OUR DISCUSSION THEY MENTIONED THAT THE FILL VALVE HANDLE; IF LEFT OPEN; WOULD CAUSE LACK OF PRESSURE; BUT THAT THE SVC DOOR ITSELF WOULD NOT BE CLOSED IF THAT WERE THE CASE. KNOWING WE WERE ABOUT TO LOSE COM I ASKED MAINT TO LET DISPATCH KNOW IF THEY HAD FURTHER IDEAS. A MESSAGE SOON CAME FROM DISPATCH WHICH WAS INCOMPLETE SO I HAD COMRDO PATCH US THROUGH AND FOUND THAT MAINT CTLR WAS CONCERNED ABOUT A LEAK OUTSIDE OF ACFT AND THAT I MIGHT CONSIDER RETURNING TO ZZZ. I FELT THIS WAS THE MOST PRUDENT ACTION I COULD TAKE. WE WERE ABLE TO GET A CLRNC THROUGH COMMERCIAL RADIO FOR A DSCNT TO FL310 AND A L TURN DIRECT ZZZ. I WENT AHEAD AND DECLARED AN EMER EVEN THOUGH WE WERE OPERATING NORMALLY. WE LANDED ABOUT AN HR LATER WITH NO PROBS. EVEN THOUGH A MECH TOLD ME THEY HAD FOUND THE HANDLE PARTIALLY OPEN THE LOGBOOK DID NOT REFLECT THAT CONDITION. I DO NOT REMEMBER THE EXACT SIGNOFF. ONE NOTE -- OUR FLT ATTENDANTS HAD NOT MADE COFFEE NOR USED WATER FOR ANY OTHER REASON UNTIL WE REACHED ALT; SO WE HAD NO REASON TO SUSPECT A PROB EARLIER IN THE FLT. HAD PREFLT PROCS INCLUDED A CHK FOR WATER PRESSURE WE MIGHT HAVE SAVED THE COMPANY THE THOUSANDS OF DOLLARS THIS INCIDENT UNDOUBTEDLY COST. AT ANY RATE THE PROB WAS RAPIDLY FIXED AND AFTER PUSHBACK WE MADE SURE THE R ENG WAS PRESSURIZING THE WATER SYS AND WE WERE AGAIN UNDER WAY.