  We present the detection of four rotational emission lines of water vapor,
from energy levels Eu/k= 101 - 454 K, in the gravitationally lensed z=3.9 QSO
host galaxy APM08279+5255. While the lowest H2O lines are collisionally excited
in clumps of warm, dense gas (density of hydrogen nuclei n_H=(3.1 +/- 1.2) x
10^6 cm^-3, gas temperature T_g ~ 105 +/- 21 K), we find that the excitation of
the higher lines is dominated by the intense local infrared radiation field.
Since only collisionally excited emission contributes to gas cooling, we
conclude that H2O is not a significant coolant of the warm molecular gas. Our
excitation model requires the radiatively excited gas to be located in an
extended region of high 100 micron opacity (tau_100 = 0.9 +/- 0.2). Locally,
such extended infrared-opaque regions are found only in the nuclei of
ultraluminous infrared galaxies. We propose a model where the infrared-opaque
circumnuclear cloud, which is penetrated by the X-ray radiation field of the
QSO nucleus, contains clumps of massive star formation where the H2O emission
originates. The radiation pressure from the intense local infrared radiation
field exceeds the thermal gas pressure by about an order of magnitude,
suggesting close to Eddington-limited star formation in these clumps.
