As you have found the literature has contrasting reports. I think it will depend on what you are trying to do, if you have a large virus that is clonal then it probably possible to optimize conditions for accurate counting. For a population of viruses of different sizes and genome sizes such as in natural seawater samples , I am skeptical that you can accurately count all viruses within a sample.
I'm am an hydrogeologist, so I have not experience in specific tecniques to detecting and quitifying virus in groundwater. But you can ask to my colluegue Antonio Bucci, He is a microbiologist.