Security is actually the first priority, and comes even before water. In common survival venacular, the first priority is shelter, however, in an urban setting, it is security. Survival priorities are based around; "What's going to kill me first". In outdoor survival (isolated from society and people) other people isn't the threat, thats the solution! And the thing that will kill you the quickest usually is exposure (hypothermia/hyperthermia), these can kill you in a matter of hours. Thus, SHELTER is the first priority in outdoor survival. But in urban SHTF or grid-down survival,  other people is what could kill you first, thus security is the first priority. You have a home, so shelter is taken care of, and next comes water just like outdoor survival. *Note, these aren't hard and fast rules that the survival police are coming for you, they are principles to keep in mind to help you prioritize. For example, if you get lost coming back from hiking, and it was warm in the day but now its approaching the 50's and you aren't making it home tonight, fire is probably your best option because it meets you thermal needs to survive the night. Makes sense?
 
So now that you have a basic understanding of survival priorities, and you know that you will die from other people before dehydration, so protection from more immediate threats come first.