We actually believe that there is much more water, because oceans are shallow compared to the Earth’s radius.
The featured illustration shows what would happen if 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.
Earth’s freshwater. Spheres showing:
(1) All water (sphere over western U.S., 860 miles in diameter)
(2) Fresh liquid water in the ground, lakes, swamps, and rivers (sphere over Kentucky, 169.5 miles in diameter), and
(3) Fresh-water lakes and rivers (sphere over Georgia, 34.9 miles in diameter).
Credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); Adam Nieman.
How even this much water came to be on the Earth and whether any significant amount is trapped far beneath Earth’s surface, remain topics of research.