The H _ { 2 } O abundance of a planetary atmosphere is a powerful indicator of formation conditions .
Inferring H _ { 2 } O in the solar system giant planets is challenging , due to condensation depleting the upper atmosphere of water vapour .
Substantially warmer hot Jupiter exoplanets readily allow detections of H _ { 2 } O via transmission spectroscopy , but such signatures are often diminished by the presence of clouds of other species .
In contrast , highly scattering H _ { 2 } O clouds can brighten planets in reflected light , enhancing molecular signatures .
Here , we present an extensive parameter space survey of the prominence of H _ { 2 } O absorption features in reflection spectra of cool ( T _ { \mathrm { eff } } < 400 K ) giant exoplanetary atmospheres .
The impact of effective temperature , gravity , metallicity , and sedimentation efficiency is explored .
We find prominent H _ { 2 } O features around 0.94 \micron , 0.83 \micron , and across a wide spectral region from 0.4 - 0.73 \micron .
The 0.94 \micron feature is only detectable where high-altitude water clouds brighten the planet : T _ { \mathrm { eff } } \sim 150 K , g \gtrsim 20 ms ^ { -2 } , f _ { \mathrm { sed } } \gtrsim 3 , m \lesssim 10 \times solar .
In contrast , planets with g \lesssim 20 ms ^ { -2 } and T _ { \mathrm { eff } } \gtrsim 180 K display substantially prominent H _ { 2 } O features embedded in the Rayleigh scattering slope from 0.4 - 0.73 \micron over a wide parameter space .
High f _ { \mathrm { sed } } enhances H _ { 2 } O features around 0.94 \micron , and enables these features to be detected at lower temperatures .
High m results in dampened H _ { 2 } O absorption features , due to H _ { 2 } O vapour condensing to form bright optically thick clouds that dominate the continuum .
We verify these trends via self-consistent modelling of the low gravity exoplanet HD 192310c , revealing that its reflection spectrum is expected to be dominated by H _ { 2 } O absorption from 0.4 - 0.73 \micron for m \lesssim 10 \times solar .
Our results demonstrate that H _ { 2 } O is manifestly detectable in reflected light spectra of cool giant planets only marginally warmer than Jupiter , providing an avenue to directly constrain the C/O and O/H ratios of a hitherto unexplored population of exoplanetary atmospheres .