We often think of the Earth as a watery planet because water covers so much of the surface, and evidence of water on Mars or our Moon excites so much notice that one might suppose it to be rare on other worlds.
In fact, however water is a very common substance, and some bodies in the outer solar system, such as comets and some moons, appear to be made mostly of water, usually in the form of ice (often called “water ice” by astronomers to distinguish it from other frozen substances, such as carbon dioxide ice (called “dry ice” here on Earth) or ices of various hydrocarbons.
Jupiter’s moon Europa is an interesting exception. Only a bit smaller than our own Moon. Europa is probably made mostly rock with an iron core (more or less like Earth, in fact), but a considerable amount of evidence suggests that it is covered with a deep ocean of water warm enough to be in liquid form, though frozen over with a thick surface of ice. The ocean depth is thought to average on the order of 100 kilometers. For comparison, even the deepest parts of Earth’s oceans are only about a tenth as deep. In fact, Europa’s ice layer alone is probably at least as deep as the Earth’s deepest oceans.
This means that the total amount of water on Europa is two or three times as much as that on Earth (!), and since it’s liquid water, it’s quite possible that it could be home to some form of life.
In fact, Europa isn’t unique in this respect; other ice-covered moons may have liquid water oceans underneath, if not so huge as Europa’s.
Here’s a panel discussion on Europa broadcast last year on NASA TV: