[smoyer]

You're both right ... the water is still in the system but no longer in the aquifers (instead it's in the oceans, air, lakes, streams, etc). Apparently, a small amount of water is lost to space (but that will increase as the sun grows hotter). When another extinction-level comet smashes into the earth, it will replenish the water we've lost.

Ain't I a ray of sunshine this morning?

[adrusi]

Lots of comets burn up in the atmosphere. The water from that also ends up on the surface. No need to have one actually impact.

[coldtea]

What water is that?

[tekromancr]

The massive pockets of ice that tend to form on giant space rocks. Comets contain a lot of ice.