I used to be an army medic. Usually when one of the guys was unable to hold down water, he had become dehydrated. More importantly they had lost their electrolyte balance. So the first general course of action? to give them ringers lactate. Which we jokingly called "IV Gatorade". that really was what it was, aside from flavoring and coloring.
Now just to make a point, it IS possible to go to the other extreme as well, but it is really actually unlikely and hard to do. Also I would like to point out that Gatorade / ringers lactate isn't a substitute for water. It is a tool to re-balance the body into accepting water.
Use this as a general rule to determine if and if so how dehydrated you are. If you are well hydrated your mouth will never feel cottony stick to the roof thirst.(This is a sign of dehydration.)And your urine will be almost colorless or pale yellow.(Think lighter than lemonade.)Be aware you can be dehydrated and still absorb water.(And even I am often now that I'm out of the Army... too much caffeine... drives out water...)
As for stock, also rick in electrolytes (very light levels of sodium, think slightly lighter than sea level.) Too much salt pulls water out of your system. (dehydration) Too much your body is 'flooded' and cant do it's job. (hyponatremia)
I would suggest talking to your family doctor. If this is persistent there may be something that he can do. That's what Doctors are GREAT at doing! solving problems. Medics just do a quick patch and keep you running, lol.