Is 12 AWG ground wire adequate for a 60 Amp water heater circuit?
To the best of my knowledge, which has been failing lately, you can't get 6/2 NB w/ #6 ground or with a #12 ground. What you can get is 6/2 NB w/ #10 ground which would be OK. I would verify that your ground wire is really #12. My guess is that it's #10. Take a piece of it into an electrical supply store and they can verify what size you have.
There are no safety concerns with a #10 ground being used in this circuit. Enjoy your new water heater.