Water piping is not a ground but needs to be bonded along with all other metallic piping systems in a building. A copper water service main is allowed to be used as a grounding electrode like a ground rod or plate.
 No piping is allowed function as an equipment ground. 
Water heaters are supposed to have ground jumpers and dielectric unions to stop electrolysis. 
Grounding and bonding are very misunderstood even by many electricians. It took a Mike Holt seminar to get a good grip on the reasons for bonding and grounding. http://forums.mikeholt.com/

 Anybody doing electrical work should have an Ugly s reference guide.

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