I saw this done quite a bit in the sixties and seventies when high compression engines were driven by little old ladies and carboned up badly.
There are TWO risks with this:
You MIGHT break off a piece of carbon that lodges uder a valve or does damage in some other way.
You might get too much water in a cylinder. Remember water does NOT compress.
I personally would not try this old trick on any of my cars. I wouldn't even do it with my 25 year old farm truck.