I'm taking my physical model of the Earth to be this sphere with a pole sticking out of it that is the height of Mt. Everest i.e. 8.8km from sea level. The water that already exists I'm assuming is below the surface of this sphere (where the surface represents sea level). The water that already exists is apparently 1.338 x 10^9 km^3. So it's just a matter of finding out the volume between the surface of the sphere and the top of the pole (if you accept my assumptions).
"It is the mark of an educated mind to be able to entertain a thought without accepting it" ~ Aristotle
