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.

Good luck,