The Earth is the planet of our solar system where almost 70% of it is covered by water that makes up the oceans. This is due to the combination of the profusion of the chemical in the solar system, hydrogen, which is the most abundant element in the universe and oxygen, one of the most common in nuclear reactions that occur within the stars. To this is added that the atmosphere created in the surface conditions temperature and pressure close to the triple point of water,i.e., the thermodynamic state which makes possible the simultaneous presence of a compound into its three states: solid (polar ice caps and glaciers), liquid (water of the seas is salty, the sweet rivers, groundwater and the drops that form clouds in the lower atmosphere), gas (steam water which is present in the atmosphere).
Although at first look ours look like a "water planet" actually makes up only a limited part of the ground volume. The ice, fresh water and water pollution, are practically small a fraction of total volume.
It should be noted that water, constantly switch from one state to another is taking place and the so-called hydrological cycles. The ocean water absorbs a significant fraction of solar radiation, especially in the red and infrared spectrum, that is how we see the distinctive blue of the oceans, that seeing Earth from space gives the appearance of a point pale blue. Sea water, heated by the sun, is a tremendous reservoir of heat that diffuses into the lower atmosphere and reduces the temperature variations between night and day and between seasons. The temperature differences between zones at different latitudes, causes large ocean currents like the Gulf and cause the water to freeze ice forming perennials.
In the sea surface, water evaporates and the vapor condensed in the air, forms clouds that are so prominent in the great pictures we get from space probes, space telescopes and astronauts. The condensation is of great importance because it is determinant in the variation of atmospheric temperature at altitude and the dynamics of the winds and meteorological systems.
The rains are responsible for distributing the continental freshwater, detail essential for life on earth parts emerging from the oceans. However, a portion of the rainwater, evaporated again and goes back to the atmosphere, the other hand, absorb plants and all other living organisms.
The continental water erodes and disintegrates the rocks, and is modified the surface of the continents. This material is taken to the seas, there are sediments that accumulate and cemented and become sedimentary rocks tend to re-emerge forming new islands or mountains.
Water vapor contributes to the greenhouse effect, the same, through billions of years prevents our planet is covered in permafrost. Also, icy surfaces, snow and clouds, have great power reflecting back to space to an interesting part of sunlight, thereby preventing the warming over the globe.