Context. The far-IR/sub-mm spectral mapping facility provided by the Herschel-PACS and HIFI instruments has made it possible to obtain, for the first time, images of H_{2_{O emission with a spatial resolution comparable to ground based mm/sub-mm observations.Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of this program, presenting a PACS map of the o-H_{2_{O 179 μm transition obtained toward the young outflow L1157.Methods. The 179 μm map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin water observations of the same source that allow us to constrain the H_{2_{O abundance and total cooling. Results. Strong H_{2_{O peaks are localized on both shocked emission knots and the central source position. The H_{2_{O 179 μm emission is spatially correlated with emission from H_{2_{rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH_{3_{. A strong H_{2_{O peak is also observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 μm intensity and its intensity ratio to the H_{2_{O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, having a H_{2_{O abundance of the order of 10^{-4^{. This testifies that the clumps have been heated for a time long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total H_{2_{O cooling is ~10^{-1^{, about 40% of the cooling due to H_{2_{and 23% of the total energy released in shocks along the L1157 outflow.}}}}}}}}}}}}}}}}}}}}}}}}}}}}

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