Water, like many substances, contains two kinds of energy. The first kind of energy is called kinetic energy. This is energy that is used during the execution of processes, such as movement. Because of kinetic energy water can flow and waves can exist.
But water can also contain potential energy. This is energy that is stored in the water. Stored, but not used. This energy can become useful when water starts to flow. It will be transferred to kinetic energy and this will cause movement.