My dog is always thirsty...?

I have a dog, he's a two year weimaraner. He weighs around 95-100lbs, however he is not overweight. He has a large body structure, with lots of muscle. Anyway, moving on. He is always drinking water, and I've become quite worried. Hes never done this before. He needs to be let out every other hour now, because of him drinking so much. Does anybody know what this could be?