Bare copper wire under kitchen sink
Carefully inspect your Grounding Electrode System

This shouldn't be it.   The Grounding Electrode System is a fat copper ground wire that goes from your service panel to any or all of: 


Ground rods outside your house.  Code calls for 2 rods 8' long. 
an "Ufer" ground that ties into the reinforcing rod in your poured concrete foundation
A cold water pipe where it enters the house, on the street side of the meter.  The problem is the utility could replace the meter with a remote-read meter, which are plastic.  


Once it's proven out that the Grounding Electrode System is tip-top... 

...then any other ground wires aren't such a big problem.  

It is likely the purpose of this ground wire is not to ground the panel to actual earth (the Grounding Electrode (GES) ... But to ground an appliance to the panel (the Equipment Safety Grounding Conductor, EGC). 

That is to say, maybe that ground wire isn't there to send fault current through the water heater, but rather to prevent fault current from a troublesome hot-water-connected device (cough dishwasher cough) from going through the water heater.