Water deficit induces a range of morphological and physiological changes in plants, triggering multiple adaptive responses. The aim of this study was to evaluate the morphological, physiological, and postharvest responses of mini watermelon plants subjected to two foliar silicon (Si) doses and three soil water tensions during two cycles. The experimental design was in randomized blocks, in a greenhouse, with a 2 × 3 × 2 factorial scheme, with four replications. Soil water tension affected plant growth, and silicon reduced plant length by 17.5%. Fruit traits were affected by the soil water tension, with silicon influencing fruit length, weight, and rind thickness. Transpiration declined from 4.06 to 1.21 mmol m⁻² s⁻¹ in cycle 1 and from 2.79 to 1.28 mmol m⁻² s⁻¹ in cycle 2 as soil water potential decreased from −30 to −65 kPa. Biochemical parameters respond to soil water tension, with total soluble carbohydrates increasing by 75% in cycle 1 and 378% in cycle 2 as soil water tension decreased from −30 to −65 kPa in the absence of silicon application. Silicon application promoted higher total soluble carbohydrate accumulation under severe water deficit conditions. In conclusion, soil water tension was the main determinant of crop performance, while silicon showed limited effects as a drought-mitigation strategy.