Context: Existing SWAS observations and future HIFI/Herschel data require a clear sense of the information content of water emission and absorption lines. Aims: Investigate wether the ground-state transition of ortho-H2O, 1_(10)-1_(01), at 557GHz can be used to measure the column density throughout an interstellar cloud. Methods: We make use of a multi-zone escape probability code suitable for the treatment of molecular line emission. Results: For low abundances, i.e., X(H2O)=<10^(-9), the intensity of the 1_(10)-1_(01) transition scales with the total column of H2. However, this relationship breaks down with increasing abundance, i.e., optical depth, due to line trapping and -- for T_(dust)>=25K, X(H2O)=<10^(-8) and n~10^4 cm^(-3) -- absorption of the dust continuum. Conclusions: An observed decline in intensity per column, expected if H2O is a surface tracer, does not necessarily mean that the water is absent in the gas phase at large columns, but can be caused by line trapping and subsequent collisional de-excitation. To determine the amount of water vapour in the interstellar medium, multiple line measurements of optically thin transitions are needed to disentangle radiative transfer and local excitation effects.