Staying hydrated in winter is just as important as staying hydrated in summer. Staying hydrated keeps warm blood pumping around your body. In winter it's just harder to do, it's cold, your fingers are numb and either your hydration-bladder tube has frozen solid or you can't be bothered to dig that bottle out of your pack where you've stashed it to keep it from freezing.
In the summer I use Platypus bottles. Ridiculously light, pack down to nothing and are perfectly tough enough unless you kick them around the campsite like a football. Unfortunately in winter their narrow openings freeze up quicker than my ageing laptop trying to run two programs at once. In winter I switch to a Nalgene Wide Mouth Soft Cantene (48oz/1.5l 50g) for water storage. Like a Platypus bottle it rolls up to nothing when empty and is tough enough to store water in my pack, next to my back where my body heat keeps it from freezing. Even when it's not in my pack the 'Wide Mouth' opening ensures that I can almost always get some water out of it, especially if I store it in my shelter upside down so any ice forming overnight forms in the bottom of it.
For drinking during the day I use a Nalgene Wide Mouth 500ml bottle. Left behind long ago by 'ultralighters' for 3-season kit these tough but relatively heavy (90g) little bottles are great during winter. I use the 500ml size as it enables me to keep it in a chest/A-line pocket of my soft-shell jackets. This keeps the bottle close to hand (so I actually use it) and close to my body, preventing freezing that can occur if you leave your drinking bottle in your pack pockets. When I empty it I simply fill it up from the Soft Cantene that's nestled in my pack. The bottle has a few other tricks up it's sleeve. It doubles nicely as a warm beverage mug in camp for trips where weight and space restrictions mean you have to leave your beloved anodised insulated mug at home. It can also be used as a hot-water bottle. Filled with hot water just before bed and nestled next to your femoral artery this little bottle will keep warm blood pumping down to your toes all night. And, if you need to answer the call of nature in the middle of the night and you don't want to leave your snuggly cocoon then the 'Wide Mouth' opening means it's easy to use as a pee bottle...
What!? It's 95% water and sterile!
