It cools down quicker because it has a lower specific heat. Radiation from the sun warms the sand and the water. However, more heat is needed to change the temperature of a material with a high specific heat.
NOTE: Specific heat-the amount of heat needed to raise the temperature of 1 kg of that substance by 1 *C.
This is because sand has a lower heat capacity than water. Water must have 4.184 joules of heat for the temperature of 1 gram of water to increase by 1 degree Celsius. Sand only requires .290 joules of heat in order for the temperature for 1 gram of heat to increase by 1 degree Celsius.
The matter that has a higher specific heat capacity takes more joules of energy to raise its temperature. Sand has a lower specific heat capacity than water, so sand will always change temperature, whether heating or cooling, faster than water.