AFTER LNDG AT SNA WITH FUEL FERRIED IN FOR RETURN FLT; FLT CREW OF B737 IS SURPRISED TO LEARN FROM CABIN ATTENDANT THAT ICE WAS FORMING ON THE WINGS PRIOR TO THE SUBSEQUENT DEP.

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LANDED SNA WITH 17.1 FUEL AND PREPARING FOR THE RETURN LEG TO ZZZ. CONDITIONS CLR SKIES 10 MI VISIBILITY ALT 30.01; TEMP 14/07 KTS WINDS 160 DEGS/5 KTS. JUST PRIOR TO BOARDING; FLT ATTENDANT COMMENTED THAT SHE THOUGHT ICE WAS ON THE WINGS. I CHKED AND YES THERE WAS FROST ON THE UPPER SURFACE OF THE ENTIRE WING. WITH FEW OPTIONS; WE DECIDED TO TRY TO USE POTABLE WATER HOSE TO DEICE THE WING. HAD THE WING BEEN LESS THAN FULL WE COULD HAVE ADDED WARMER FUEL AND MELTED THE ICE BUT THE WINGS WERE FULL OF SUPER COOLED FUEL THAT WAS CAUSING FROST TO FORM WITH NO VISIBLE MOISTURE ON THE GND OR ENCOUNTERED DURING DSCNT OR APCH. IT TOOK ABOUT 20 MINS TO GET THE WING ROOT AREA CLRED AS THAT WAS THE POINT OF GREATEST ACCUMULATION (ABOUT A DIME'S THICKNESS) AND I DID A CHK WITH MY BARE HANDS TO CONFIRM THE WING WAS CLR. THE REST OF THE FLT WAS UNEVENTFUL. AT NO TIME DID IT OCCUR TO ME OR THE CAPT TO LOOK FOR ICE AND A COMMENT BY THE FLT ATTENDANT SAVED THE DAY. IN MY OPINION; WE SHOULD NOT BE PLANNING TO CARRY SO MUCH FERRY FUEL TO SNA OR WE BETTER GET DEICING EQUIP OUT THERE.