Actually, fresh water is becoming a limited resource in many places and access to it is already causing conflicts.
But assuming you are talking about hydro-electric power generation, then it is regarded as a renewable resource, because due to the Sun and Gravity, water continually cycles from sea level, up to the clouds and back down via rivers, and the potential energy of the water vapour in clouds is converted into kinetic energy in rivers which can be harvested with quite simple mechanisms (Dams and turbines) to provide electrical energy, at any point on it’s fall to sea level, at which point (as well as some earlier) it will again evaporate (using Solar energy) and regain the potential energy to start the whole process again in a constantly renewed cycle.
Renewable energy is a confusing and somewhat misleading term as it really refers to constantly naturally renewed energy sources, not ones which have to be renewed. Sustainable energy is a better term as it stands up to linguistic analysis, and describes what is key in the distinction from un-sustainable ones.