  We have conducted a study combining H2O lines in two spectral regions. First,
Infrared Space Observatory observations of several H2O thermal lines seen in
absorption toward Sgr B2(M) at a spectral resolution of 35 kms^-1 have been
analyzed. Second, an IRAM-30m telescope map of the para-H2O 3_13-2_20 line at
183.31 GHz, seen in emission, has also been obtained and analyzed. The H2O
lines seen in absorption are optically thick and are formed in the outermost
gas of the condensations in front of the far-IR continuum sources. They probe a
maximum visual extinction of ~5 to 10 mag. Radiative transfer models indicate
that these lines are quite insensitive to temperature and gas density, and that
IR photons from the dust play a dominant role in the excitation of the involved
H2O rotational levels. The water vapor abundance in the region is (1-2)x10^-5.
The relatively low H2O/OH abundance ratio in the region, 2-4, is a signature of
UV photon dominated surface layers traced by far-IR observations.
