The oceans represent a massive reservoir for liquid water. Whether through direct precipitation upon the surface of the ocean or through the slow process of accumulation in an aquifer and eventual deposition, most water molecules will eventually end up here and spend the vast majority of their existence as part of a surface reservoir.
Though this reservoir represents the majority of liquid water on the Earth, the water is in a form that is unusable to terrestrial life because it is saline, or salt water. The processes of evaporation or boiling are necessary to separate the water molecules from the sodium and other polluting molecules.
