A B757 PILOT IS INFORMED BY FLT ATTENDANT THE CABIN POTABLE WATER WAS INOPERATIVE. ACFT SHUDDERED AT CRUISE ALTITUDE. LATER FOUND THE RIGHT INBOARD SLAT WITH VERTICAL TEAR AND WATER PANEL DOOR JAMMED SHUT.

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WE HAD AN UNEVENTFUL TKOF AND CLB TO CRUISE. WHEN THE SEATBELT SIGN WAS TURNED OFF; THE FLT ATTENDANTS CALLED TO SAY THE POTABLE WATER WAS INOP. WE HAVE NO PROCS AIRBORNE TO FIX THAT PROB AND TOLD THEM THERE WAS NOTHING WE COULD DO UNTIL WE GOT ON THE GND IN ZZZ. WE WERE CRUISING AT FL380; PAST THE FIX ZZZZ IN THE MIDDLE OF OCEAN WHEN THE ACFT SHUDDERED AS IF WE RAN THROUGH SOME WAKE TURB OR JET WASH. ALL INSTS WERE INDICATING NORMAL AND THE PLANE APPEARED TO FLY NORMALLY. THE FLT ATTENDANTS CALLED AND ASKED ABOUT THE DISTURBANCE. WE ASKED IF THEY HAD JUST MOVED A BEVERAGE CART AS THE SOUND WAS SIMILAR TO ROUGH HANDLING OF A BEVERAGE CART. THEY SAID THEY HAD NOT. WITH EVERYTHING APPEARING NORMAL; WE ASSUMED IT WAS SHIFTING CARGO OR A POCKET OF TURB. THE FO MADE A PA AND TOLD THE PAX THE DISTURBANCE WAS TURB. THE FLT CONTINUED ON WITH AN UNEVENTFUL DSCNT; APCH; LNDG; AND TAXI IN TO THE GATE. BEFORE THE FO COULD DO A PREFLT FOR THE NEXT FLT; A NON-REVENUE PLT TOLD US SHE SAW A PIECE OF METAL TORN ON THE SLAT. THE FO AND I WENT OUT TO THE R WING AND SAW A VERT TEAR IN THE ALUMINUM OF THE INBOARD R SLAT. THE DAMAGE WAS CLEAN; WITH NO BIRD DEBRIS. THE ZZZ GND CREW TOLD US THE POTABLE WATER DOOR WAS IMPROPERLY SECURED; JAMMED SHUT. ON OUR PREFLT INSPECTION OUT OF ZZZZ; THE POTABLE WATER DOOR WAS SECURED AND NOT LEAKING. THERE ARE NO CTLS FOR THE POTABLE WATER IN THE COCKPIT. THE POTABLE WATER DOOR; FUSELAGE AND SLAT CANNOT BE SEEN FROM THE COCKPIT. A ZZZ RAMP AGENT WAS ABLE TO CALL AND I WAS ABLE TO USE HIS PHONE TO RELAY THE INFO. THE REPRESENTATIVE WANTED US TO LOOK OVER THE ENTIRE AIRPLANE AGAIN TO CHK FOR ADDITIONAL DAMAGE. WE DID THAT AND THE SLAT WAS THE ONLY DAMAGE. ZZZZ ARPT WANTED OUR GATE SO THE ACFT WAS MOVED. THE FO AND I WERE IN THE COCKPIT COORDINATING THE MOVE WITH GND CTL. WE GOT BACK TO OUR ORIGINAL GATE; MET WITH OUR FLT ATTENDANTS; AND WERE INFORMED THE COMPANY WANTED US TO LAYOVER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE POTABLE WATER TANK WAS SERVICED PRIOR TO DEPARTURE. GROUND CREW AT ARRIVAL STATION SAID THE WATER SERVICE PANEL WAS LEAKING AND SERVICE VALVES NOT PROPERLY CLOSED. SOMEONE HAD MANAGED TO JAM THE DOOR PANEL SHUT; BUT WATER WAS STILL LEAKING OUT. REPORTER STATED THE BELIEF IS; THE LEAKING WATER PANEL DEVELOPED AN ICE CHUNK THAT SEPARATED IN CRUISE AND STRUCK THE SLAT CAUSING THE VERTICAL TEAR.