Why does water take long to heat and long to cool down

Water is a slow conductor of heat which means it needs more energy than land or air to to increase it's temperature. That means once heated it will hold on to that heat much longer.

Water take long to heat and cool down is because it needs to gain more energy than a comparable amount of air or land to increase its temperature, and it means that, once heated, a body of water will hold onto that heat for a much longer period of time than either air or land.