I've browsed the majority of posts on this thread.
During my photographic life, I have lived in Kent, East London and now Essex. I too have had water stains on SOME strips of negatives over the years.
Whilst in London, I bought a middle priced photographic water filter. which stopped any 'stains' appearing, but a main part broke and I then started to use bottles of 'de-ionised water', bought from the local garage. I used it as my final wash with just a drip or two of photo-flo added to it. Again no stains.
I had a disabling accident just before moving to Essex and for a while continued to use bought water, when able to buy some. Then, after reading another thread about this issue, it was recommended to use a home water filter. I have and use a Brita system. Using tap water filtered through this, in the last two washes, provides me with clean negatives, comparable to buying and using the shop bought water!
If you have a home water filter, try it. If not, at least think about it, as various systems can be bought relatively cheaply. Others have mentioned using the water collected by home 'dehumidifiers', with great success, so worth a try if you already have one.
Terry S