There's something called rain, and it's what makes rivers flow, actually. They don't magically sprout out of a rock. Well, most of the times, anyways :p The process is simple: it rains, water gets to a river, river gets to sea, water from sea evaporates, but the salt doesn't. Repeat as many times as needed, like the last million years. And if you need it desperately, there are other means, like suggested before me.
Edit: AND ALSO! Water can be chemically produced on the atmosphere: It's H2O, and hidrogen and oxigen are abundant in our atmosphere. They sometimes react. And you know what? Water can chemically be divided into Hidrogen and Oxigen, by using electricity, for example. The balance always stays, so don't worry about it. Electric storms are usually dry, guess why?
Edit: AND ALSO! Water can be chemically produced on the atmosphere: It's H2O, and hidrogen and oxigen are abundant in our atmosphere. They sometimes react. And you know what? Water can chemically be divided into Hidrogen and Oxigen, by using electricity, for example. The balance always stays, so don't worry about it. Electric storms are usually dry, guess why?