Yes you can use water and other soap to clean your windshield when you run out of windshield washer fluid however during the winter months the water and soap could freeze causing your windshield washer fluid tank to freeze and break.
If you use water and regular soap in your windshield washer fluid reservoir be sure to add some rubbing alcohol to the water and soap mixture to prevent your windshield washer fluid reservoir from freezing and breaking in cold weather.
And yes if you're running low on windshield washer fluid you can top your windshield washer fluid off with some water although it may not clean your windshield as good when it's diluted.
Also be sure to add some rubbing alcohol to the windshield washer fluid as well if you do add some water to it to prevent it from freezing too in the cold weather.