With real estate agents moving away from doing water tests in rural areas, I have added it to my list of services and my clients are happy. So are the realtors. In real estate, time is a very hot commodity. If I do the water test when I'm doing my inspection, it saves everyone time and money (depending on the lab).
Water testing is not rocket science but you do have to be very particular about how you do it. I always use lab provided bottles, alcohol swabs to wipe the faucet, and a hand sanitizer on my hands to eliminate any chance that I will be the source of micros.
Comments from my clients and realtors have been very positive and it's a great source or revenue for me. Win - win.