I carry two one litre drink bottles in my top box, and have a decent guzzle whenever I stop, usually every couple of hours. I discussed hydration with my doctor, and on his advice I put a couple of teaspoons of gatoraide powder in each bottle, which speeds up the absorption of the liquid into your body, and reduces the overfull feeling after a big drink. Weak cordial, some fruit juice, or virtually any additive to the water gives the same benefit.