by 
gooong
ANSWERS: 4
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Sand is a solid and a poor conductor of heat. That means that when sunlight hits sand, all the energy of the sunlight is absorbed in the first millimeter or so of the sand, the heat stays there or spreads only a few millimeters down. So those few millimeters get quite hot. If you dig down, you will find the sand remains cool underneath, Water is partially transparent, so that the sunlight penetrates quite a distance into it. So, on average, the sunlight is absorbed in the top meter of the sea, rather than the top millimeter. But then the sea is usually being stirred by the waves, so the warm water is mixed with cold drawn up from underneath. if the day is very calm indeed, you can sometimes feel a warm layer at the top of the water, about 15 centimeters deep. But usually the heat is mixed in to the mass of the sea so that you just feel a tiny warming of the top few meters of the sea.
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Although i agree that the answer provided by ImAlec is very informative however it is incomplete in 2 respects: 1. It does not explain that the sand at night cools down faster than the water. 2. Sand being solid the heat transfer process is through conduction and water being liquid the heat transfer process is through convection. As is easily understood conduction is a faster process than convection. (eg. heating a solid rod of iron on a flame is faster than heating it using hot air/gas). Another factor which plays a very important role here is the heat capacity of a substance. Heat Capacity (amount of heat required to raise/lower temperature by 1 degree) of water is much higher than that of soil. Hence it warms up slower and cools down slower than sand. Hope finally this completes the good answer provided by Mr. ImAlec
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Actually, whilst the average heat capacity plays a large part, and there is about a trillion times more ocean than beach sand, none of those are the primary cause of the superheating effect.( meaning it really does get hotter than bare rock) It is the same effect that cause ice to be soft under snow yet not in the sun. The insulative capacity of the "Air" between the particles keeps the heat locked out, the same as the snow keeps the cold out thus making ice below it weak. Air is the greatest known insulator, and beach sand is one of those great natural displays.
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Many reasons but one is sand does not evaporate like water, therefore you do not benefit from its cooling effect. The water also has greater area to disperse the heat.
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