Yet water vapour floats in cloud layers and ozone in a layer up to 50 kilometres high.
Water vapour is actually less dense than air.
It is liquid water at sea level that is roughly 800 times more dense. However, liquid water droplets and solid ice crystals do form clouds but they do not float. Because the droplets and crystals are so small, and therefore fall very slowly, they are effectively carried by the wind and, to a person on the ground, they appear to float. Ozone too does not float. The density of an ideal gas at a particular pressure and temperature is proportional to its molecular mass.