Under my list of must-haves, I include "means of purifying water". This is because you can go for weeks without food. You can survive being shot. You can survive all kinds of things. But nobody can survive long without water. All of those other "nice to have" things won't do you any good if you die of heat stroke or dehydration. Therefore, this may be the most important section with regard to staying alive for the short term. Getting water after a natural disaster can be extremely difficult, because water service is often interrupted, or the water provided is not safe to drink

Hydration needs differ greatly depending upon your situation, but the basics are the same. The world is mostly covered in water, but you can't actually drink much of it. Most fresh water can be made drinkable and rain can be collected. Dry places can have all kinds of hidden water. Sea water can be distilled. Understanding different ways to procure good water is more important than having the best gear.

Drinking bad water won't usually kill you, but it can make you so sick you wish you were dead. Even a relatively minor illness during a survival situation, when medical attention may be days or weeks away, can be devastating. Safe water is the key to staying healthy.

• Water purification - Clearly, boiling water is the simplest way to turn bad water into OK water. Of course, boiling water isn't always an option. Do you want to try to get a fire going when everything is soaking wet, the wind is blowing at 50mph, and there are groups of looters roaming the neighborhood? Me neither. Since hydration is my #2 short-term survival concern (after immediate self defense), I like to have at least a few options available. Obviously a means of making fire (or a stove of some kind) and a metal container to boil is important. I also always keep chemical purification tablets (iodine) with my gear. Liquid chlorine also works in a pinch. You can chemically treat water while you are moving, simply fill up your Nalgene, drop in a tablet and keep walking. Backpacking filters (like the Katadyn Hiker) are also nice to have because they are usually the fastest way to make “wild” water potable. Just keep a backup handy, because filters can break, freeze or become clogged. A few companies also make a water bottle with a built-in filter. These only barely work, but are good a pinch if you have to grab wild water and keep moving. If it is an option, I like to use a backpacking filter with a activated charcoal component. In addition to making the water taste better, the charcoal in the filter can trap some nasty chemicals in questionable water. Most mechanical water filters only remove dirt, leaves, bug parts, and larger bacteria.
• Nalgene bottle - You have to have a good water bottle. Being able to filter or boil water is great, but if you can't carry water with you, then your mobility is severely limtied. There are a lot of water bottle options out there. I prefer a wide-mouth Nalgene type bottle. It is less likely to freeze closed than a narrow-mouth bottle, and easier to fill with boiled water from your cook pot. They are also great storage containers for things that shouldn't be crushed, or that need to stay dry. Of course, I've also never had a problem using a PET plastic soda bottle. They are light weight, rugged, and easy to find.
• Hydration bladder* - I have one of these in my frame pack. I use it for backpacking. In a survival situation where constant mobility is paramount, a CamelBak type hydration bladder can keep you moving with your hands free to attend to other needs (like your rifle, for instance.) Still, I don't trust mine as a primary water container in a survival situation. They are not as robust as Nalgene bottles, or even soda bottles, and seem like they would be more prone to leaking and failure. If you have one, go ahead and carry it, as they are really convenient, but have a backup.