An un-named British subsidary of a large American helicopter company operating out of Warri, Nigeria had a problem with engines on S-76's. Seems they were changing engines faster than they did socks....which knowing some of our engineers would not mean much. Tear down analysis of the engines showed corrosion equivalent to old boat anchors raised from the sea floor after centuries.
Seems the "tap water" was a bit saline....and over a few months was enough to do in brand new engines. I wonder how many engines died an untimely death due to that small oversight?
I would suggest distilled....sparkling clear....pure...pure water for engine washes....any doubt....have your tap water tested before using it. You might be surprised at what you are feeding your engines.