  Molecular electronics is a fascinating area of research with the ability to
tune device properties by a chemical tailoring of organic molecules. However,
molecular electronics devices often suffer from dispersion and lack of
reproducibility of their electrical performances. Here, we show that water
molecules introduced during the fabrication process or coming from the
environment can strongly modify the electrical transport properties of
molecular junctions made on hydrogen-terminated silicon. We report an increase
in conductance by up to three orders of magnitude, as well as an induced
asymmetry in the current-voltage curves. These observations are correlated with
a specific signature of the dielectric response of the monolayer at low
frequency. In addition, a random telegraph signal is observed for these
junctions with macroscopic area. Electrochemical charge transfer reaction
between the semiconductor channel and H+/H2 redox couple is proposed as the
underlying phenomenon. Annealing the samples at 150{\deg}C is an efficient way
to suppress these water-related effects. This study paves the way to a better
control of molecular devices and has potential implications when these
monolayers are used as hydrophobic layers or incorporated in chemical sensors.
