The water pump shown on the left is to replace the old one shown on the right. This particular waterpump is driven by the timing belt, you should always replace the water pump when you replace the timing belt. The old water pump will not likely last the lifetime of the new belt.
At the point in time when you replace your timing belt your waterpump may seem fine, but if you put a brand new belt on an old waterpump it is not likely that the waterpump will last for the life of the new timing belt.It is well worth that extra labor that it takes to replace that pump, believe me!