This chapter explores the essential role of water in regulating Earth’s environmental systems. This chapter provides an in-depth examination of the hydrosphere, which encompasses all forms of water on Earth, including oceans, rivers, glaciers, and atmospheric water vapor. It highlights the critical functions of the hydrological cycle in maintaining water balance and sustaining life. Furthermore, this chapter discusses the significance of ocean waters, their temperatures, salinity, and the role of ocean circulation in climate regulation. Special attention is given to current challenges facing the oceans, including the impact of climate change, pollution, and overfishing. The dynamics of water’s role in Earth’s climate, ecosystems, and human societies are analyzed, emphasizing the need for sustainable water management and environmental protection.