Drinking water is extremely important in the cold. Often times your natural thirst is absent because you are cool, making it more likely for dehydration to set in. In addition to the typical dangers of dehydration, it can accelerate frostbite, and is an attributing cause of hypothermia. Avoid eating snow because it can lower your core temperature.
Frozen water is dead weight. Here are a few tips for keeping your water drinkable on winter hikes or overnight trips.
The obvious tip is to prevent freezing in the first place. A frozen nalgene requires a lot of time, energy and effort to thaw because you cannot apply heat directly. The primary preventative measure is to store your bottle in a place that will best prevent freezing (in your pack, close to your body or in your jacket, sleeping bag, etc.). Placement inside your pack is better than an outside pocket, though it may be more cumbersome to access. Don't let the inconvenience discourage you from drinking, though.
Warming your water before putting it in the bottle will extend the time before it freezes, as well. The myth that hot water freezes faster than cold water only holds if a large amount (up to 1/4) evaporates, leaving less water to freeze. In a sealed water bottle, there is no evaporation, and it will remain unfrozen longer.
Using chemical heat handwarmers stuck to or stored with the bottle will keep your water from freezing, and may last longer than simply warming the water. Using handwarmers in combination with the wool sock insulating sleeve, described below, is a winning combination. (As an aside, sticking the warm water bottle in your sleeping bag is a great way to make it cozy.)
Finally, use Gatorade. The salts contained in Gatorade or other electrolyte drinks will lower the freezing temperature of your water. The impact may be minimal, but the changed freezing properties can make for a great tasing slushy.
When using camel backs and like products, try these recommendations:
1. Keep the bladder close to your back, or try hanging it from your neck with a makeshift lanyard;
2. Use an insulated tube;
3. Drink frequently, to keep a flow of unfrozen water;
4. After each drink, blow the water back into the bladder, so it is not in the tube where it can more easily freeze, clogging the tube; (Thanks 96avs01!). Repeat after taking your bag off; the tube may refill with water.
5. Shove the tube inside your jacket (through the collar or pit zips).
6. Consider removing the bite valve, which can quite easily become frozen.
Even though most bladders are designed to withstand freezing (the bladder/tube won't burst), under truly cold conditions, the tube will freeze and so they are not an ideal choice. You're probably better off with a nalgene bottle, so save the bladder for warmer weather.
To improve your chances of being able to drink water that is freezing in your bottle, always place it upside down. Your bottle will freeze from the top down because ice floats, and the ice that forms will rise to the top. This phenomenon is particularly notable in an agitated bottle, as is the case when stored in your backpack. With a bottle that is always stored upside down, even if the bottle starts to freeze, you can access the liquid portion from the capped end. The only caveat is to keep the cap tight!
By the way, Nalgene bottles will withstand freezing (they won't crack or burst, even if full of water before freezing).
Before you throw away that mismatched wool sock, consider saving it for the noble purpose of extending the length of time your water will stay liquid.
The simple method is to double up the sock and slip it over the nalgene. Fold it in a counterintuitive way, with the top of the sock folded inside; this makes for a tidier insulating sleeve (the two can be compared in the image). The insulation will help prevent freezing in your pack, and is quite useful if you have your bottle in your jacket, to insulate the water from the portion of the bottle pressing against your outer shell.
A craftier way is to assemble the insulater as follows. Cut a 1/2-inch slit horizontally in the sock (reinforce with thread or duct tape if you like), at the location where the sock layers are at the top of the bottle. An ideal location for the slit is the heel, where there is some exceess, non-symetrical material. Remove the cap ring from the bottle, squeeze it in half and insert it through the slit (see Image 2). Now the sock is hanging from the cap loop, with the cap and ring on either side. Slide the sock over the bottle, replace the ring, and the sock will stay in place (see Image 1). The socks can also serve as a spare pair of dry socks or mittens in a pinch.
Commercial solutions are readily available (see comment section below).
TNF has a patent for a heating element to prevent the tube from a water bladder from freezing. I wonder if they will be able to develop something that makes it to the market. The patent can be viewed here: