For many years the extent of water emission and its abundance in molecular clouds have been key problems in the study of the interstellar medium -ISM-. Although all chemical models predict a high water abundance, ground-based observations of non-masering rotational transitions of water are practically impossible. The first detected 22 GHz transition of water, which has been observed in many star forming regions, circumstellar envelopes and external galaxies, is in fact a maser emission which probes small volumes of gas characterized by extreme physical conditions. Recently, Cernicharo et al. (1990, 1994) succeeded in observing at 183 GHz the weakly masering 313-220 transition of water in many molecular clouds opening a new window to study water in the ISM. In particular, it was shown for the first time that water emission in Orion is spatially extended. The derived abundances demonstrate that water is one of the most abundant molecular species in warm molecular clouds. However, the masering nature of the 313-220 transition limits its use as a diagnostic of the physical conditions of the gas and the excellent weather conditions required to observe at 183 GHz clearly restricts its measurement. ISO is the only instrument in the next decade which will allow to observe at high spatial resolution and sensitivity the thermal emission of water vapor in the ISM. The angular resolution of ISO in the far-infrared together with the sensitivity of the LWS FP spectrometer will enable to map and resolve the large scale distribution of water in nearby molecular clouds. We propose to map several nearby molecular clouds in two lines of H2O connecting low-lying energy levels. Other transitions of water and its rare HH18O isotope will be made towards selected positions in these molecular clouds. The proposed observations will provide fundamental data on the distribution of water in molecular clouds and will result in a major contribution to our knowledge of water and its role in the interstellar medium.