I'm not sure that Opti-Coat Pro Plus should be considered a magic bullet against hard water. You've got to dry before the droplets do. I don't know how many gallons of ONR you mix up, but distilled water is only a buck a gallon from the supermarket, I would use that as cheap insurance.
I would talk to your installer about how best to remove the water spots, but it may be that MDR is indicated. Have you tried simply re-wiping the area with your (wet) wash media and redrying?