  Interstellar ices are largely composed of frozen water. It is important to
derive fundamental parameters for H$_2$O ice such as absorption and scattering
opacities for which accurate complex refractive indexes are needed. The primary
goal of this work is to derive ice-grain opacities based on accurate H$_2$O ice
complex refractive indexes and to assess their impact on the derivation of ice
column densities and porosity in space. We use the \texttt{optool} code to
derive ice-grain opacities values based on new mid-IR complex refractive index
measurements of H$_2$O ice. Next, we use those opacities in the
\texttt{RADMC-3D} code to run a radiative transfer simulation of a protostellar
envelope containing H$_2$O ice. This is used to calculate water ice column
densities. We find that the real refractive index in the mid-IR of H$_2$O ice
at 30~K is $\sim$14\% lower than previously reported in the literature. This
has a direct impact on the ice column densities derived from the simulations of
embedded protostars. We find that ice porosity plays a significant role in the
opacity of icy grains and that the H$_2$O libration mode can be used as a
diagnostic tool to constrain the porosity level. Finally, the refractive
indexes presented here allow us to estimate a grain size detection limit of
18~$\mu$m based on the 3~$\mu$m band whereas the 6~$\mu$m band allows tracing
grain sizes larger than 20~$\mu$m. Based on radiative transfer simulations
using new mid-IR refractive indexes, we conclude that H$_2$O ice leads to more
absorption of infrared light than previously estimated. This implies that the 3
and 6~$\mu$m bands remain detectable in icy grains with sizes larger than
10~$\mu$m. Finally, we propose that also the H$_2$O ice libration band can be a
diagnostic tool to constrain the porosity level of the interstellar ice, in
addition to the OH dangling bond, which is routinely used for this purpose.
