The "drink lots before" method works OK for me on short rides, but on longer, rough rides it won't cut it. I hit a "wall" of total exhaustion on my last major ride a few weeks ago that was likely the result of dehydration. Even in extreme cold you still sweat a lot while cylcing, you just don't notice it esp. with modern wicking fabrics. Plus the conditions and the larger tires make progress more difficult. For me it was running into a five mile stretch of rime ice that slowed me way down and eliminated any cruising. After that I was exhausted and stuck well away from any quickee marts.
Obviously you can't drink sea water. But I'm going to start packing some extra liquids in the form of salty broth, and maybe even bring my little compact cook stove to warm it up. Even a heating candle and a tin can would work. It sounds a bit extreme but I never want to go through that experience again. I felt like a member of Scott's doomed expedition to the South Pole. A little hot tea and lots of water when I got home brought me right back to life, though. I just need to be able to do that on the trail. Hot water with some cubes of broth in it would go down well. Or some miso. Yum.