I wouldn't pour water on the windshield, personally. What I do when there's frost/ice on my windows is start the engine, set the heater on defrost, and the fan on high. If I have a rear window defroster, I also turn it on at the same time as I start the engine. Then, while I'm waiting, I brush the snow off the vehicle, and grab my (PLASTIC!!) scraper. I start scraping on the side windows. Do all of those, then move to the back window. Once that's done, the windshield has softened enough to be able to scrape it.
Will scraping the frost/ice off the windows scratch them? It can. You can even break the window, if you hit it too hard in the wrong place. But, it's either that, or idle your engine forever and a day while you wait for the frost to melt.