First thing you need to do in this scenario. Be it an EMP attack, cyber attack, or an inside job, is to plug up all the sinks, baths and get anything that can hold water and fill them ALL up. All your pots, pans, jugs, gas cans, cups, coffee pots, vases, EVERYTHING fill it to the brim before the water turns off.
I have a useful habit that I developed several years ago when I ended up without electricity for a couple of weeks due to an ice storm.
I live on the shore of a lake and as soon as it freezes, I gut a 3x3 foot hole in the ice and keep it open all winter. I throw a piece of plywood over it and go out and break what little ice forms every day. It will come in handy for flushing toilets or washing and if worse comes to worse, cooking.
Filling the bathtubs used to be the number one order 50 and 60 years ago, for hurricanes, for anything.
Amazon.com has a “waterBoB” that fits in the bathtub and holds 100 gallons of water for drinking, $20. I’ve got one on order but have no first-hand experience with it yet. But it gets good reviews.