You might be right, I can appreciate the homeowners concern in that respect, but unless I am misunderstanding the basic requirements for the depth such pipes have to be, the homeowner should have no concern that a detectorist would dig anywhere deep enough to even get close.
I just did a search for such guidelines in Nebraska (the OP's home state) and it seems that water mains need to be at least 5 feet deep -
https://www.mudomaha.com/sites/default/files/1.11.2.pdf
Now again you could still be right about the homeowners concern because he might have no idea of the big difference between how deep the water main is and how deep a detectorist might dig.