Are you constantly incredibly thirsty? Coffee isn't actually a net loss in water - as Zerofries suggested, something else is wrong, or you're drinking a lot of beer all the time(which I'd argue is probably also "something wrong").
One can dehydrated not in a sense of having not enough water, but in the sense of not having enough salts to hold the water in body. In this case, the water you drink just becomes urine very fast and doesn't quench thirst at all. Just taking a bit of bare salt is not a solution either, you have to get some kind of an oral rehydration solution (either a specialized one, an isotonic drink, or maybe eat some soup if your apetite is not impaired). More about this here: https://en.wikipedia.org/wiki/Oral_rehydration_therapy and here: https://en.wikipedia.org/wiki/Water_intoxication
Yeah, they make special salt packets you can use called Oral Rehydration Salts that have an osmolal ratio of electrolytes plus just enough glucose to cause your upper intestine to rapidly take the fluids in. If you are severely dehydrated drinking a liter of the stuff is enough to restore you to sanity within about half an hour. I usually carry one or two packets in my hiking/climbing first aid kit. Trioral is a really common brand.
Mountaineering is strenuous and all-day enough that you drink water all day, and hyponatremia is a serious concern from drinking straight water for some people, like me. So what I normally do is mix up some Nuun and drink that every few hours. If I get in a bad way and run completely out of water. then I will mix up a liter of ORS and down that. But that's an emergency kind of thing for me because one of the side-effects I've noticed is that ORS will make you thirsty sometimes.