How much of planet Earth is made of water?
Very little, actually.
oceans of water cover about 70 percent of Earth's surface, these oceans are
shallow compared to the Earth's radius.
The above illustration shows what would happen is all of
the water on or near the surface of the Earth were bunched up into a ball.
The radius of this ball would be only about 700 kilometers, less than half the radius of the
Earth's Moon, but slightly larger than Saturn's moon
Rhea which, like many moons in our outer Solar System, is mostly water ice.
How even this much
water came to be on
the Earth and whether any significant amount is
beneath Earth's surface remain topics of research.