Coughing at nightime & after drinking water.

Question
My dog is almost 10, he coughs after drinking water and during the nighttime.  I have to let him out in the night and then give some ice cubes, he can breathe better after that and goes back to sleep.  The X-rays showed nothing and our vet doesn't know what is causing the problem.  What can we do to stop the coughing?  Would a cool mist humidifier help at all?

Answer
First of all, I think I might get a different vet as yours didn't seem to try too hard to solve the problem, and a second opinion never hurts.

This sounds like it could possibly be a collapsing trachea, and I didn't have to search more than a few seconds to find that information.
http://www.vetmed.wsu.edu/ClientED/cough.aspx
http://www.veterinarypartner.com/Content.plx?P=A&A=1527
http://lowchensaustralia.com/health/tracheal.htm

If this is the reason for his coughing, things you can do that might help are weight loss if he is overweight, some sort of air filter (stop smoking, if someone in the house smokes), medications, cough suppressants, airway dilators, and possibly surgery. All medications should be done under a veterinarian's direction.

And that second link indicates that liver function tests should be run in the case of a collapsing trachea as lack of oxygen can cause liver disease in some dogs, and those dogs might need further supportive medications.