Hydrology is the study of the Earth's water cycle (hydrologic cycle, the placement and movement of water on Earth including water vapor, ice, etc.). The term is also applied to similar study of other planets and moons, and by analogy, to other cases where some other compound is capable of being solid, liquid, and gaseous on a body.
About 0.02% of the Earth (including the atmosphere) is water. The balance required for a planet to retain water can involve the typical atmospheric water cycle (vaporization and condensation) and a deep water cycle, of the water sitting in the crust pushed into the mantle at subduction zones, then re-released where mantle material is exposed, at ridges, rifts, and volcanos. The Earth has all this. Under the pressure of the mantle, silicates can transform water into a form that holds the hydrogen and oxygen of water in the form of OH⁻ ions. The cycling depends upon the temperature and pressure conditions of the upper mantle, and models suggest it could work up to 5 Earth-masses, which becomes an interesting limit in evaluating extra-solar planets for their chances of harboring Earth-like life.
The terms water planet and ocean world are used for planets of considerable water: they may be Earth-like, or covered with water, perhaps with ice underneath. Earth serves as a useful model regarding interior processes (such as the deep water cycle) if at least 2% of a body's surface is land. Atmospheric spectral signatures of such structures are of interest, e.g., the ratio of methane and carbon dioxide, which may reveal whether there is ice under the ocean.