I would change it. Most of the labor charge will be already in the timing belt labor - so you are getting the water pump for the cost of the part and a very small amount of additional labor. If you don't get it replaced and it needs replacement in the near future, the labor charge will be much higher and you will kick yourself for not having it done with the timing belt.
I think that the interval for replacing the timing belt is 105K miles. I know a dealer who claims that when you adhere to this schedule, you might get away with changing the water pump every other timing belt change. Maybe, with 105K intervals in few years, with all highway driving, but it doesn't seem worth the risk to me.