BTDT. A waterbed works fine for emergency water. Just know, the polypro beds will have a heavy plastic taste. That being said, that plastic leaching is not a problem at all for flushing toilets and or washing. You'd have to make other arrangements for drinking water which is actually far less water.
The better mattresses, if you can find one, are the nylon. They're softer to the touch too but what in context, they leach far less. They're about twice the cost but worth it but in reliability and comfort.
Treatment, simply use bleach and if the bed is used, the natural motion will keep the bleach mixed.
Not everyone has the space to store barrels etc. In fact, I was living in an apartment in TX when I had to use mine for water after hurricane Alicia. A King, it was 200 gallons which was more than enough to flush our toilet, wash dishes, taking a bath, etc. for a week and still not bottoming out the bed at night.
There's absolutely nothing wrong with separating your water storage by usage, potable and non-potable, and actually a better approach often.
Tj