Most people end up getting their water pump replaced whenever they get their timing belt replaced. This is because in most cars the water pump must be removed in order to access the timing belt. It is often easier and necessary to just replace it with a new one rather than to put the old one back in.
I remember when this first happened to me I thought the mechanic was trying to trick me. I needed a timing belt and here he was trying to sell me a water pump too. But it is really a good idea to get it replaced at the same time because if you are getting your timing belt replaced you have probably put enough miles on your car that it is time to do the water pump too. Trust your mechanic on this one. They know what they are talking about.