Replacing a water pump and getting the cover to seal, without taking the head off, is very difficult, even with the engine out.
I would not even attempt to work on the water pump in situ, as you have to disconnect the timing chain sprockets, then get them lined up again when re-building, apart from getting the water pump seal correct.
It is very easy to drop something down into the sump, maybe, or stuck in the path of the timing chain maybe.
Gynaecologists might attempt this, but I think most mechanics would take the engine out.