I cross-country ski in the winter, usually not more than 2-3 hours, but it can get quite cold, -15, -20°C, and I'm often on a frozen lake exposed to wind that rapidly rips the heat out of anything.
In those conditions, I cannot simply leave a bottle in an outer pack -- they freeze, particularly once they are half empty, and it is easy to end up with a good proportion unusable. I like to take ~1-1.5 liters and consume most of it.
Since I'm travelling light, there isn't much to put in a pack except water, so nothing to insulate it with. Even if I throw in an extra fleece, this is not enough.
The best solution I've found is to wear a camelback style reservoir beneath my outer layer in a flimsy drawstring backpack. However, the hoses on those freeze very easily, and even with it under a jacket I have gotten enough ice in the line to make it hard to use, etc. It's also a bit awkward, and I dread having the bag or the hose burst or leak.
Anyone have any better ideas? It would be great if there were something palatable I could add to the water to lower the freezing point.