Regarding die-electric fittings: they also break the electrical continuity through the water heater. The cold water line would have been required to be bonded to the main electrical system ground. This continuity was assumed to be continuous through the water heater to the hot water piping. The die-electric unions break this continutiy and now the hot water piping needs to be bonded to the cold water piping (which we will assumed is already bonded).
This should be done by a qualified and licensed electrical contractor who should also verify that the cold water lines are properly bonded.