  Above-treshold ionization (ATI) by femtosecond laser pulses centered at
515\,nm is studied for a gas mixture containing the Van-der-Waals complex
Ar-H$_2$O. By detecting photoions and -electrons in coincidence, the ATI
spectra for Ar, Ar$_2$, \HHO, and Ar-\HHO are discerned and measured
simultaneously. Using an intensity-scanning technique, we observe the red-shift
of the ATI spectra as a function of the laser intensity. The
intensity-dependent shift of the ATI peak positions observed for Ar-H$_2$O and
H$_2$O match but significantly differ from the ones measured for Ar and Ar$_2$.
This indicates that the photoelectron is emitted from the \HHO site of the
complex and the vertical ionization potential of Ar-H$_2$O is determined as
$(12.4 \pm 0.1)$\,eV. For resacttered electrons, however, an enhancement of
high-order ATI is observed for Ar-H$_2$O, as compared to H$_2$O, suggesting
that the relatively large Ar atom acts as a scattering center, which influences
the ionization dynamics.
