Earth may or may not have been formed with water, but if it did, it was all baked out by the sun when Earth was just a kid. All the water we have now comes from a time when Earth was bombarded with comets, which, as you know, are basically big balls of that gross dirty snow you see in the city a few days after plowing. We're all drinking space water!!

Source: Astronomy Cast, episode 64: Pluto and the Icy Outer Solar System