Windshield washer fluid is more than just water.
Windshield washer fluid contains a cleaning detergent as well as ingredients such as 99% isopropyl alcohol which keeps the windshield washer fluid from freezing in the winter.
Windshield washer fluid isn't that expensive so it's much better to use windshield washer fluid instead of plain water.
However if you must top off your windshield washer fluid with some water be sure to put some rubbing alcohol in with the water to prevent it from freezing in cold weather.