"so you are saying water can prevent other medical conditions which have dehydration as a symptom? well, i'm glad to know you are an expert."
No, I'm not. I'm sorry to know you struggle with English comprehension. The ad did not say (and I did not say) that water could prevent all medical conditions leading to dehydration. It said it could reduce the risk of dehydration. This statement is true PERIOD.
Let's take an example. About once a year in the US, some high school kid dies during sports workouts because it was hot and he wasn't drinking enough. Not, he had a condition. Not, he was drunk. Just, it was hot and he wasn't drinking enough water. Drinking more water would _almost certainly_ have saved such the kid's life. In particular, it would have reduced the risk of dehydration.
Now, did the ad say, "Water will guarantee no dehydration?" No, of course. Neither does ANY drug or substance guarantee it will work in the face of every condition that may cause the symptoms it addresses. They can only address RISK, the average over the population of the different causes and factors causing the condition.
And guess what, in a very large proportion of cases, water does indeed stave off dehydration.
"I said: 'if the dehydration is being caused by a clinical condition, and water will not fix it, then the claim is false.'
with which part are you disagreeing? "
I'm disagreeing with the relevancy of the statement, since the claim was not that water fixes dehydration in every case.
"(i believe that dehydration is still a major source of child mortality in the developing world, where clean water supplies may be more difficult to find and 'common sense' says you shouldn't put water in a leaking pot, as it will just leak more... so parents of children with diarrhea do give them more water... "
Why would clean water supplies matter? Surely there's enough water in their food?
"and if 'water hydrates' is common in sense enough as everyone seems to claim, then what is the need to say it? "
That is a completely silly statement.