All water on the planet exists in a cycle called the water cycle, so none is ever really lost.
Water in the seas and rivers and lakes is evaporated when it gets warm by the sun (like when a kettle turns water into steam). This rises up and forms clouds.
when the clouds get blown over mountains or get colder again, the condensation turns back into water again and then it rains. The rain flows into rivers and lakes and the sea again and the process starts all over again!