  Recent observational studies have shown that H2O emission at (rest)
submillimeter wavelengths is ubiquitous in infrared galaxies, both in the local
and in the early Universe, suggestive of far-infrared pumping of H2O by dust in
warm regions. In this work, models are presented that show that (i) the
highest-lying H2O lines (E_{upper}>400 K) are formed in very warm (T_{dust}>~90
K) regions and require high H2O columns (N_{H2O}>~3x10^{17} cm^{-2}), while
lower lying lines can be efficiently excited with T_{dust}~45-75 K and
N_{H2O}~(0.5-2)x10^{17} cm^{-2}; (ii) significant collisional excitation of the
lowest lying (E_{upper}<200 K) levels, which enhances the overall
L_{H2O}-L_{IR} ratios, is identified in sources where the ground-state para-H2O
1_{11}-0_{00} line is detected in emission; (iii) the H2O-to-infrared (8-1000
um) luminosity ratio is expected to decrease with increasing T_{dust} for all
lines with E_{upper}<~300 K, as has recently been reported in a sample of
LIRGs, but increases with T_{dust} for the highest lying H2O lines
(E_{upper}>400 K); (iv) we find theoretical upper limits for L_{H2O}/L_{IR} in
warm environments, owing to H2O line saturation; (v) individual models are
presented for two very different prototypical galaxies, the Seyfert 2 galaxy
NGC 1068 and the nearest ultraluminous infrared galaxy Arp 220, showing that
the excited submillimeter H2O emission is dominated by far-infrared pumping in
both cases; (vi) the L_{H2O}-L_{IR} correlation previously reported in
observational studies indicates depletion or exhaustion time scales,
t_{dep}=Sigma_{gas}/Sigma_{SFR}, of <~12 Myr for star-forming sources where
lines up to E_{upper}=300 K are detected, in agreement with the values
previously found for (U)LIRGs from HCN millimeter emission...
