<p>One in four people worldwide suffers from freshwater scarcity, amplifying the need for sustainable desalination technologies. Among them, double-slope solar still (DSSS) offers a simple design structure and enhances distilled water production through natural evaporation–condensation principle. To reduce the reliance on costly and time-consuming experimental set-ups, empirical correlations have been developed to represent natural convection heat transfer within solar stills, a key factor influencing the thermal performance and freshwater yield. However, the validity of these correlations for DSSS systems remains uncertain. This study thoroughly analyses existing natural convection correlations, using available experimental data from various DSSS set-ups. Significant discrepancies are observed between model predictions and measured values, with temperature errors reaching 6.69–7.61% for glass cover and 6.11–10.50% for water surface, leading to severe deviations in freshwater production. Maximal underprediction reaches 90.24%, corresponding to Ali et al.’s model, with the highest overprediction of 357.28%, corresponding to Dunkle’s model. To address these inconsistencies while maintaining the original formulations of the models, a physically plausible correction methodology is proposed. The corrected models show significant improvement, with MAPE decline to the range of 8.07–61.12% and achieving the coefficient of determination up to 0.9875. Although this correction approach considerably enhances the applicability of existing models, it is not intended to replace the development of generalized natural convection formulations but rather highlights the limitations of existing ones and the need for more refine correlations that specifically reflect the internal mechanisms of DSSS systems.</p>