Is there such thing as a national or statewide GIS data set for existing sewer and water lines?
There is a program designed to prevent accidental destruction of buried utilities, which has made the effort to provide data about underground utilities to the public. This is the "Call before you dig" (http://call811.com/) program. Each state has it's own version of the "call before you dig" program. This free program will send a person out to a planned dig site to mark the ground with spray paint where there are buried utilities. The user experience is pretty low-tech, but there does appear to be geospatial source data. I wasn't able to figure out how to get access to that source data. I haven't fully investigated, but I suspect it's proprietary.
Here's the information I was able to glean from my recent experience with this program.
I made a "design request" for a single parcel with the Ohio program (http://www.oups.org) and they emailed me diagrams of the property with its nearby buried utilities. The information about each utility (gas, water, sewer, electrical, etc) came separately, and the quality varied from utility to utility. For example, here's what they sent me about sewers:
Several planning diagrams, apparently scanned from paper copies of 100-year old diagrams. Some of the streets in these diagrams don't exist today, and it's not clear if those streets were ever actually built. Given that, I'm not sure how accurate the sewer locations themselves would be.
A pdf of a map that was clearly produced in a GIS software, with line and point data overlaid on a basemap with streets names and aerial imagery. The source data must have been a vector layer, presumably digitized from those original diagrams.
The data was sent to me by a private company, One Call Concepts Locating Services (occls.com). That company obviously has geospatial data of the buried utilities. This quote from their website leads me to believe that they will not provide that data to you.
Our proprietary records management software, PlatView, allows us to search and review customers' records upon arrival at a locate request site. PlatView can use the actual GPS location of the vehicle to search for the correct utility records, providing the most efficient use of the technician's time on site.
If their business revolves around locating buried utilities (for a price), providing the source data would make them obsolete.
The free "call before you dig" service only provides information about public utilities. The only way to know where private utilities (pipes, wires, etc) are buried on private property is if all the previous property owners kept good records and passed them on to the current property owner. Obviously that information is not publicly available. You can hire a service to do a site visit and locate forgotten buried utilities. After making my "call before you dig" request, I received several emails advertising various private utility locating services (eg Blood Hound, https://www.bhug.com/). I believe these companies use a device to locate forgotten buried utilities, like this:
TL;DR: Geospatial data about water and sewer pipes exists, but it seems to be proprietary.