Dry mouth is a common side effect of low-carb, high protein diets in general. You don't have to be in ketosis to get it, although since ketosis is also caused by low-carb and high protein diets, ketosis and dry mouth tend to go hand-in-hand.
Incidentally, so is bad breath and, for many people, a metallic taste in the mouth. In addition to drinking water, you might try chewing sugar-free gum. Unless you're chewing multiple packs a day, the carbs in it are negligible, and won't stall or slow your weight loss. (That being said, some people swear that sugar-free products make them crave sugar, making diet adherence harder.)