I like to use my Fix+ with MSFs and MBs (just my Colour Craft ones, I don't think Sonic Chic blushes need this). I also use it with pigments and MES (mostly with my Style Black ones).
I really don't think Fix+ could be substituted with water. Fix+ has glycerin in it, which is known for its hydrating qualities, and if I would to spray water on my face, my face would dry out instantly. It's like licking your lips when they're chapped. At least on my face anyway because it's prone to dryness, can't say how water would work for ppl with oily skin.
However I'm sure there are a lot cheaper alternatives for Fix+, the ingredients don't seem to be expensive or complex at all.