Some officers carry bottled water and emergency food rations, if you work in a desolate or even a rural area, that's probably not a bad idea, I'm fortunate to work in an area where 24-hour convenience stores are everywhere.
But during bad weather or when preparing for extended operations, such as sitting on an extended crime scene, I'll usually stop by one of them and pick up some snacks and drinks, more for personal comfort than a matter of survival.
As far as the bag itself, a number of decent manufacturers make suitable bags. The one 1 use has a removable pocket flap on top so you can remove it to sew on a patch or have it embroidered. I had our unit patch and a black nametape with my last name on it sewn on. This makes it a little easier to identify.