Earth is like dough mixed to a precise recipe: two parts water to one part land. Too much water, and the planet becomes a vast ocean where intelligence has nothing to grasp onto. Too much land, and water retreats deep, leaving a desert. Scientists put this culinary analogy to the test with statistics. They compared four hypotheses about what water-to-land ratio is most common among habitable worlds. The balance model turned out to be the most plausible: it explains why we exist. Extremes, water worlds or dry planets, are almost incompatible with the emergence of an observer.
To reliably distinguish between scenarios, we need to study about a dozen habitable planets around other stars and measure their water budgets. For now, Earth is the only cookbook, but soon we'll be able to peek at alien recipes.
Modern Mars is a dry desert, but billions of years ago, an ocean likely splashed across up to a third of its surface.
In Lem's 'Solaris,' the ocean planet thinks — an extreme where water became intelligent without a single patch of land.