Here's the Physics we need in order to answer the question:
The specific heat of water is . . . 4185.5 Joule per kilogram per °C.
That's IT ! Everything else is arithmetic.
Volume of the pool = (10m) x (5m) x (1.5m) = 75 cubic meters
Density of water = 1,000 kg/m²
Mass of water in the pool = (75 m³) x (1,000 kg/m³) = 75,000 kg
Change in temperature = (100°C) - (20°C) = 80°C .
Energy absorbed by the water in the pool to raise the temp 80°C:
Energy = (4185.5 J/kg-°C) x (75,000 kg) x (80°C)
Energy = (4185.5 · 75,000 · 80) (J · kg · °C / kg · °C)
Energy = 2.511 x 10¹⁰ J
Energy = 25.11 GigaJoules
But honestly ... the event that caused your swimming pool to get uncomfortably hot was the beginning of a nuclear war, so you probably have more important things to worry about right now.
