Your concern isnt totally unfounded in my opinion,here are my thoughts.
1) In the event that copper water line is actually one that leads underground (NOT just internal not in earth piping) I would consider it is a readily available GroundING Electrode and make it a part of an entire GroundiNG Electrode System, where it as well as other approved Grounding Electrodes, such as driven into the earth copper rod or rods (made electrodes) are all tied together and to which the incoming Neutral GrounDED Conductor is bonded (i.e. earth grounded)
2) In the event its NOT a Suitable GroundING Electrode (cant bond it to the driven rods and the Neutral) and one was concerned if a hot wire were to get shorted to it a shock hazard results, if it were connected to the panels Equipment Ground Buss by an Equipment GroundING Conductor (the bare/green wires) then if a live hot wire got shorted to the pipe, the Equipment GroundING Conductor would serve as a fault current return and trip the breaker thereby de energizing the live hot voltage hazard.
NOTE AND CAUTION a) I'm confident of Para 1 as NEC proper, I always used all readily available GroundING Electrodes such as buried utility pipes and driven rod or rods etc etc b) I'm confident the "THEORY" of Para 2 is correct, i.e. if the pipes bonded to the equipment ground and a short resulted such would trip the breaker......... c) HOWEVER I HAVE NO RECALL OR KNOW WHAT THE NEC WOULD SAY ABOUT MY PARA 2??? (if that pipe is NOT a suitable GroundING Electrode) I'm headed to Church and no time and besides I'm too lazy to look it up lol There are plenty of knowledgeable current practicians electricians onboard who are much more familair with the latest codes then myself SO NO WARRANTY ON PARA 2 (nor even 1 for that matter, its only my rusty code opinion)
See yall after Church
Ol John T and all