I assume your water pump is leaking? Or are you replacing it for some other reason?
Anyway, assuming that the water pump needs to be replaced, then there's no choice but to remove it. If there's enough rust there that one of the bolts may break upon removal, so be it. If they all come out, great! If one or more breaks, then that's the way it goes. Your mechanic will then drill out and repair the hole using a thread insert (commonly called "Heli-Coil") and give you a bill for the additional time and materials. That additional charge if something does break shouldn't be that much, there's enough room in there for a guy to get a short bit and a 90-degree drill to the offending bolt.