Can water get in oil through water pump or timing belt case?

No, but you should, every second timing belt. All the same labor is required to change either part, and the cost of changing the timing belt or water pump is almost entirely labor cost. "While we're in there" is actually legitimate in this case.

You have to take off the serpentine belts so that you can remove the pulley for the water pump. You also have to take off the cam head so that you can get to the timing belt case and remove the plastic splash guards in front of the water pump. Remove the timing belt, then there are 4 bolts attaching the water pump. Remove those bolts and then disconnect the water intake and put your new pump in and replace the timing belt. **Make sure that your timing marks are located where they need to be before replacing the timing belt.**

It is behind the timing case cover behind the crankshaft pulley.You have to remove the timing belt to replace the water pump for this repair.

Inside the timing case with the timing belt and balance shaft belt.

Buick regal doesn't have a timing belt. It has an accesory belt and is change between 50,000 to 60,000. In my case i replace the accesory belt when my car was 80,000 milles.