If you carry your water in water bottles in winter, it’s going to freeze unless you take precautions. It’s easy to get dehydrated in winter unless you force yourself to drink liquids because you lose moisture with every breath and don’t notice when you’re thirsty. Furthermore, resupply can be difficult if all the water around you is frozen solid as ice and snow unless you melt it with a stove.
Before you go hiking, boil your water to make it really hot. Then place it inside a wool sock inside your backpack and surround it with the insulated clothing items you carry in your pack to keep it warm. You can also purchase a neoprene or foam water bottle parka and pack it in that instead.
If you carry a bottle of water on the outside of your backpack in a neoprene or foam insulating sleeve, turn it upside down. This will prevent the cap from freezing shut. Don’t use a hydration pack/hose or metal water bottles. They freeze up and are impossible to use afterwards.
In addition, don’t use Nalgene bottles with narrow necks in winter. Use wide mouth bottles, preferably white-mouth polyethylene bottles that can “take” rapid temperature changes (pouring boiling water into a very cold bottle) without cracking. The best wide mouth bottles are called Hunerdorf or Relag bottles and have an untethered top that is easy to unscrew even if you’re wearing mittens.
If you are camping in winter, boil your water before retiring and sleep with the bottle(s) in your sleeping bag to prevent them from freezing.
Winter Hiking, Hydration, and Insulated Water Bottle Jackets
Cold Weather Water Treatment and Purification