the water pump can be changed whenever ,not driven by the toothed timing belt
however...
you may as well while you have the aux belt off anyway(4 bolts) ,and , buying just a pump on it's own will cost £25+ ,so cheaper to buy a kit with toothed belt,water pump,deflection roller and tensioner and do it once ,
i went for a gates kit myself but that's about £5 more if you pick the right one (complicated as it's listed as corsa )
On the V6 Omegas I think forum policy has changed about changing water pumps with cam belts. On the in line fours, with the cam belt using the water pump as a pulley it may still be policy to change the pump with the cam belt.
If the pump is driven by the cam belt, then it's
always part of the belt change. Doesn't matter what the engine is or whatever it's fitted to.
If the pump isn't driven by the cam belt, then it's discretionary.
The original OOF suggestion to change the V6 pump with the belt was for preventative maintenance because a genuine GM pump was very cheap. They're no longer cheap, so the suggestion is that it makes more financial sense to leave it until fails. Which
ought to be a very long time. Of course, you don't have to buy a genuine pump
Personally, I prefer not to disturb parts unnecessarily