I believe the younger generations have moved past this but all old people (particularly 80 years of age or older) start most sentences with "back when I was younger.....................". Also, there is a difference between military 'training' where water is readily available and an actual war where you are stuck in the middle of nowhere with little rations in the heat surrounded by the enemy. I dare say in those situations the soldiers are not just running off to the water coolers every 5 minutes. Also, I dont believe letting people run off to get a drink is teaching them about hydration. When I trained at a club with regular drink breaks the students put no effort whatsoever into preparing for a martial arts class, they would just roll up dehydrated, start the class and just get a drink every now and then. Where I train now there are no drink breaks so the students have learnt the importance of hydration as a form of 'preparing' for exercise, they have become pro active rather than re active. On the days I do tkd I start hydrating myself from the minute I get out of bed right up to the minutes before class starts. Its only an hour, its not like we are running an olympic marathon.
Personally, I carry a camelbak and at least two canteens anytime I'm outside the wire. Most do and some carry more. Long convoys, we always took a few frozen bottles of water too. In the desert, if you don't hydrate, you die.
The point that is important is that everyone is different. People are at different levels of fitness, their bodies use water at different rates. Personally, I sweat gallons when I work out. Granted, I can run a 10k without water, but I'm miserable by the end and AT the end, I guzzle water. And I can tell you that if I do stop to drink or use the electrolyte gels, my time is better by minutes. I know people that can run longer than that without even thinking about water. On a run of that length, pre-hydration doesn't matter much, because you use it up.
Plus, drinking a boatload of water beforehand doesn't necessarily ensure that you're hydrated.
Finally, if someone DOESN'T prepare properly, what training value is there in punishing them, by not allowing them to drink during class? Make them suffer, they get less out of the workout, they are less focused, and perform worse. All because you want to teach them a lesson? Isn't the lesson supposed to be martial arts?