In this study an attempt was done to compare the thermal performance for the three identical single-slope solar stills using three different depths of seawater (1, 2, and 3 cm). It was executed under the climatic conditions of Mansoura University (31.045°N latitude angle, and 31.352°E longitude angle, and 6.72 m above the sea level). The experimental work was carried out form the 1^st of January until the 19^th of April 2020. Different of climatic factors and solar still measurements were monitored, measured and recorded. The hourly average solar energy available inside the solar still wind speed and ambient air temperature during the daylight-time, respectively, was 461.4 W, 1.3 m/s and 21.4ºC. The thermal efficiency for the lowest depth of seawater was higher than that of the other two depths by 22.44% and 49.03%, respectively. The distillation efficiency of the lowest depth of seawater was higher than that of the other depths by 13.34% and 20.15%, respectively. The lowest depth of seawater augmented the productivity rate of freshwater by 42.04% and 77.27% as compared with the other two depths, respectively. The estimated costs of distilled one litre of freshwater from the three different depths of seawater, respectively, were approximately 0.68, 0.96, and 1.20 EGP.