If this is truly a metallic water system, that is the best electrode but there is no guarantee these days that there is any metal as soon as it leaves the house. I agree electricity doesn't take the path of least resistance, it takes all paths. You just get an inverse proportional current flow to each.
Ground rods tend to be the highest resistance ground electrode of the approved types. There is no requirement to even what that resistance is after you drive the second one.
