Yeah well, water is practically free, readily available, relatively benign if it leaks, and can be a hydrating life-saver in a pinch. So it's a pretty good option as something other than air.
Keep in mind that everything with some amount of sugar or salt has a thaw point below 0 degrees. Practically all of your food will have thawed by the point the ice melts - so you’re dumping a lot of energy into freezing water that will be mostly useless at the point disaster strikes.
Adding salt to your water will substantially lower the thawing point and it’s still cheap and widely available.
If you want to prep for cases when you need water you’d be better off keeping your freezer cold with a suitable medium and have a few 30liter buckets of water with some sort of additive that prevents spoilage (silver ions for example) or have some packs of bottled water around.
