Why does it take pool water so long to warm up?

Water has a very high specific heat capacity, which means you need to put a lot of energy into a given quantity of it to raise the temperature a given amount. The specific heat capacity of water is 4.187 kJ/kgK, which means you need to put in 4 kJ of energy to raise the temperature of one kg of water by 1 K (or 1C) Compare this to the specific heat capacity of air which is 1.0 kJ/kgK, or 25% of that of water. Therefore, you need to put in four times as much energy to heat up a given mass of water as compared to a given mass of air. One kilogram of air has a volume of about 29 cubic feet at 1 atmosphere, and a kilogram of water is a liter. It takes 4 times as much thermal energy to raise the temperature of that liter of water by a degree as it does to raise that 29 ft3 of air that same one degree. To do an additional comparison, you could picture a room that is 11' x 11' x 11' in volume. Elevating the temperature of the air in that room one degree will lift the temperature of that liter of water by the same one degree.