I have a Barnett pouch and recently have been waking up at night with pain (which does not feel like gas pains). I've had the pouch for 15 years, but 3 years ago I had a 1.5 inch "stone" in the pouch and even though it was removed, the pouch has never been the same.
I go to Dr. Shen at Cleveland Clinic and a few years ago he prescribed opium tincture (formerly called paregoric) for pain. It's liquid and about the worst tasting stuff in the world--but it does work for pain the in the middle of the night. Most drug stores are clueless about this medication and have to order it. It's a controlled substance so there are no refills and it has to be a printed prescription. I take .3 ml before bed and I have not been waking with pain. I still do wake up for other reasons, and then I do empty my pouch. I agree, it's not fun at 3 am.
As far as dehydration, I have been hospitalized a few times. Both times the Doctors said, by the time you are dehydrated and feel the leg cramps and other symptoms, drinking water is not going to help. So now I take potassium, magnesium and electrolyte tablets that you put in a bottle of water called "nunn" which are available at vitamin shoppe. I also found some electrolyte gummies which are easy to take when you are out and do not have a water bottle.
Good luck with your problem.
I also recently tried immodium before bed, not sure if it really works, but it does supposedly slow down the pouch.