Let's first look at our homeworld, to use as a reference. The Earth's surface water has a mass about 0.00023 times the Earth's total mass, with the planetary ocean having 96.5% of it. The ocean has a mean depth of 3.8 km, and they cover 71% of the Earth's surface, giving a planetwide average of 2.7 km. I use singular, because what are usually called oceans are one continuous body of water, sometimes called the World Ocean. Turning to the Earth's interior, estimates vary widely. Water distribution on Earth - Wikipedia from Quest to trace origin of Earth’s water is ‘a complete mess’ | Science News: 1.5 to 11 oceans. Estimates of the mass of water inside Earth range from 1 O to 50 O where 1 O: 1 to 50 oceans, noting Drake & Campins (2006) estimating 10 oceans and Marty (2012) estimating 4 to 12 oceans. If the Earth had much less surface water, then most of it would be in the crust as groundwater. About 1.69% of the Earth's surface water is groundwater, so if the Earth had 1% of its present amount of surface water, then the Earth would look waterless: a desert planet. If the Earth had much more surface water, then all its land area would be drowned, making it an ocean planet. One can estimate how much water would be needed by considering the height of the highest possible mountain. From the surrounding terrain, the highest mountains on our planet are Mauna Loa and Mauna Kea at about 10.3 km. Mt. Everest is the highest in gravitational potential, but it rests on some already-elevated terrain. That makes it about 12.6 km above the average ocean floor. That means that 4 to 5 times the water would be enough. Why might the Earth have almost enough water to cover its land area, but not quite? One can try to work it out from theory, but the theory of planetary formation already has had some nasty curveballs thrown at it by observations. Curveballs like warm and hot Jovian planets. But one can look for Earth-sized habitable-zone planets, and indeed, it has been possible to find some such planets. [1704.04290] Updated Masses for the TRAPPIST-1 Planets -- though those masses have big error bars, they are enough to show that at least five of those planets are likely ocean planets: b, e, f, g, h. Planet d straddles the all-rock line, meaning that it also may be an ocean planet, and planet c is between the rock and iron lines, as Venus and the Earth are. It would be difficult to rule out a few-hundred-km-deep ocean on c, however. So the Earth's amount of water might be a rarity. Anything on other exoplanets?