|MadSci Network: Earth Sciences|
The mass of all the water on the planet is estimated at 1.35 x 10^18 metric tons. (If you're not familiar with scientific notation, please ask your teacher.) The total mass of the planet is 6 x 10^21 metric tons. This gives a ratio of 1 to 4400. This comes out to 0.023%.
As you can see this is a very small number. It makes sense, though, that it should be a very small number. The oceans cover about 7/10 of the planet, but the average depth is only 3.8 km (2.3 miles) and the diameter of the earth is 12,730 km. Also, the bulk of the planet's mass is made up of a very large molten metal core and the semi-solid mantle surrounding that, both of which are much more dense, and therefore more massive, than water.
Try the links in the MadSci Library for more information on Earth Sciences.