The evolution of sex determination is complex and yet crucial in our understanding of population stability. In ectotherms, sex determination involves a variety of mechanisms including genetic determination (GSD), environment determination (ESD), but also interactions between the two via sex reversal. In this study, we investigated whether water deprivation during pregnancy could override GSD in two heterogametic squamate reptiles. We demonstrated that water restriction in early gestation induced a male-biased secondary sex ratio in both species, which could only be explained by water sex reversal as the more likely mechanism. We further monitored some long term fitness estimates of offspring, which suggested that water sex determination (WSD) represented a compensatory strategy to ensure a balanced operational sex ratio. This study provides new insights into sex determination modes and calls for a general investigation of mechanisms behind WSD and to examine the evolutionary implications.