The hydrosphere is one of the Earth's spheres. It consists of the liquid water on and above the Earth's surface, such as the oceans, rivers, seas, etc. The hydrosphere is essential to other Earth-like planets as well.
The hydrosphere distributes water and energy throughout the Earth's oceans, "connecting" them. With the help of the wind, the Great Ocean Conveyor Belt moves enormous amounts of heat from the equator to the poles, where it is released. At the same time, the cold water from the poles cools the air at the equator keeping temperatures there less extreme. Surface currents tend to be warm; deep currents tend to be cold.
The currents are complicated by the spinning of the Earth and having to move around and past continents. Countries situated close to warm ocean currents, such as the United Kingdom, would be much colder without these warm flows of water like the Gulf stream passing their shores. The currents also bring with them whole communities of plants and animals, many traditional fisheries depend on warm currents like the gulf stream.