Dan Phiffer found this image on a message board (it's by Adam Nieman / Science Photo Library), and by his calculations, he says it's accurate.

Left: All the water in the world (1.4087 billion cubic kilometres of it) including sea water, ice, lakes, rivers, ground water, clouds, etc.

Right: All the air in the atmosphere (5140 trillion tonnes of it) gathered into a ball at sea-level density. Shown on the same scale as the Earth.