Why is water considered a good insulator?

In addition to convection being faster than just conduction, water also has high specific heat, which means its temperature does not change quickly when heat is added. ... Since water is high, it take a large amount of energy to raise its temperature. That is why it is a good insulator.

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Heat transport by convection in water (and most liquids) dominates transport by conduction. Free convection, where the change in density of water with temperature causes cold water to fall and warm to rise, is very efficient in moving thermal energy.