It is true. Although water levels drop, the amount of water on Earth remains the same - just in different forms. You should know the cycle of water: ice melts, liquid water (streams and rivers) go to the ocean, water from lakes, rivers and oceans evaporate (due to the heating caused by sun light) then the water vapor ascends and accumulates in clouds in the atmosphere (gravity attraction keep the atmosphere and do not permit the clouds escape from Earth) and when the clouds are heavy (loaded with a lot of water) and cools down, the rain occurs bringing the liquid water again to the surface.