Average depth of the ocean is several miles (don't know the exact depth, assume 3 miles.) Earth is 4000 miles in radius. Water covers about 3/4 of the surface, so that works out to a rough estimate: 1/1000 of the earth's radius is water. Volume is 4/3 Pi R^3, so as you can see, the volume of surface water is tiny compared to the volume of the Earth...
1/(1000 * 1000 * 1000) = 1/1,000,000,000 of the volume is surface water.
The 4/3 and Pi cancel out, as they are in the volume calculation for both the earth and the water.
On the other hand, it is the surface that we live on...