Perhaps you wake up dehydrated because you are not consuming any hydration while you are sleeping, unless, of course, you wake up through out the night to drink?
Having said that, I can only assume most people are dehydrated upon waking. How about drinking a large glass of water when you awake along with some celtic Sea Salt
Also, your grogginess could very well be low cortisol as many people with adrenal dysfunction have low cortisol in the am (just the opposite of what it should be).
Well theres my 2 cents worth,..
Hope this helps :)