I'm no expert, but the water pump is generally driven off the timing belt, so it makes sense to change the water pump at the same time - otherwise the timing belt service would essentially have to be repeated to change the water pump at a later time. Take a look at this thread (top photo) for VW CJAA and CBEA 2.0 TDIs and you should be able to see what I mean. I don't have any specific knowledge of the 125TDI but I'm sure someone else on here will be able to help you out.
How to install and replace new timing belt on VW Jetta TDI 2005 2006 2005.5
Cheers
Bookmarks