I have been advised that adding water is perfectly acceptable - it's only water and no different to having a shake and having a glass of water at the table at the same time as your meal. My preference is to add more water to the shakes and particularly the soups - I am not keen on them without the extra. dilution.
I've been putting more water in too - and having some of them hot, made with water from the kettle when it has boiled and rolled down a little. - makes them like a hot choc, hot strawberry etc. I like them that way.