The water shortage is because the pipes froze and burst. Also because electricity was cut to pumping stations so there was less pressure. When pressure is low, the water becomes non-potable (you can't drink it) because it sits more, so people without burst pipes still had to boil it first anyways. Low pressure is also bad for fighting fires, they have to bring out an additional tank truck because the hydrants have little flow. It isn't about a literal lack of water.