  H_2O submillimeter emission is a powerful diagnostic of the molecular
interstellar medium in a variety of sources, including low- and high-mass star
forming regions of the Milky Way, and from local to high redshift galaxies.
However, the excitation mechanism of these lines in galaxies has been debated,
preventing a basic consensus on the physical information that H_2O provides.
Both radiative pumping due to H_2O absorption of far-infrared photons emitted
by dust and collisional excitation in dense shocked gas have been proposed to
explain the H_2O emission. Here we propose two basic diagnostics to distinguish
between the two mechanisms: 1) in shock excited regions, the ortho-H_2O
3_{21}-2_{12} 75um and the para-H_2O 2_{20}-1_{11} 101um rotational lines are
expected to be in emission while, if radiative pumping dominates, both
far-infrared lines are expected to be in absorption; 2) based on statistical
equilibrium of H_2O level populations, the radiative pumping scenario predicts
that the apparent isotropic net rate of far-infrared absorption in the
3_{21}-2_{12} (75um) and 2_{20}-1_{11} (101um) lines should be higher than or
equal to the apparent isotropic net rate of submillimeter emission in the
3_{21}-3_{12} (1163 GHz) and 2_20-2_{11} (1229 GHz) lines, respectively.
Applying both criteria to all 16 galaxies and several galactic high-mass
star-forming regions where the H_2O 75um and submillimeter lines have been
observed with Herschel/PACS and SPIRE, we show that in most (extra)galactic
sources the H_2O submillimeter line excitation is dominated by far-infrared
pumping, with collisional excitation of the low-excitation levels in some of
them. Based on this finding, we revisit the interpretation of the correlation
between the luminosity of the H_2O 988 GHz line and the source luminosity in
the combined galactic and extragalactic sample.
