Interestingly, we used to use tap water daily on a previous model of the same engine series, with the full knowledge of the contracted maintenance organisation which is also an accredited agent of the engine supplier. For 8 years we never had a contamination problem despite operating low level close to the sea.
When we replaced the old aircraft with one powered by a later model of the same engine we asked if we could continue that practice and were told we couldn't; not because of the water but because it's technically more complex so chemical washes every 50 hours would be acceptable. Now that the UK fleet is showing signs of contamination the manufacturer has decided a daily wash with distilled or deionized water is the only way.
I'm not asking for advice on what to do as we can handle that side of things, I'm only interested in the general practice.
Any more observations?