Not sure what are you calling "Ground". You may "bond" the water heater to the grounding electrode conductor, but don't alter the electricians connections as this could get you electrocuted.
National Electric code says the water piping system can be used as a "grounding electrode" system if the underground metal pipe is longer than 10 feet. A grounding electrode conductor should be hooked up by the electrician within 5 feet of where it enters the building.
Even if the house is predominantly pex, It still could be used as a "grounding electrode" provided it meets the 10 foot rule. Otherwise, it still needs to be "bonded" back to the electric service.