This can be machined out to sound metal and a larger seal fitted or else a alloy ring shrunk in to restore the dimension. If its a hole through the pump cavity surface then it is more difficult. Agree that welding is not a repair option for the water pump. I have heard of people using liquid metal, Belzona etc but I haven't used it myself.
Welding is fine for cracked magnesium covers etc where there is good clean metal to weld but you need an expert welder - an aviation welding mob is who I use - not the local guy who makes BBQ's
. Had excellent results on several repairs. Cleanliness is critical and these guys know how to degrease etc.
Good luck!