The balance shaft timing chain is the Achilles heel of the G54B. Well, actually it's the balance shaft bearings. At high mileage they can seize up and snap the chain. This stops turning the oil pump, but the cam and distributor are on a separate chain so the engine keeps running. If the engine has high mileage, I would consider replacing the timing chains and balance shaft bearings, or installing a balance shaft elimination kit.
Changing the water pump is a pretty quick and easy job. You have to remove it to do the timing chains, but it's not enough work to justify doing the timing chains "because I have the water pump off"