This is due to a physical property called specific heat capacity; the heat capacity of sand is lower than that of water.
The specific heat capacity is known as the amount of energy needed to raise the temperature of 1 g of a substance 1 degree celsius.
Also, the specific heat determines who quickly the temperature of material will raise or fall when it gains or loss heat energy
Furthermore, the specific heat of water is one of the most specific heats of the substances. Therefore, liquid water requires more heat energy to increase its temperature than sand.
So, it is always found that the water is cooler than sand and also water losses its energy slower than sand that the sand be cooler faster than water after sunset.
