That is really an old practice started by older coaches (not of this generation) and I guess the logic behind it is to make the athlete more mentally tough through depravation. I don't know if it really works though. I've been on both ends and I can't say that it does or doesn't make a difference. I wouldn't worry too much about dehydration unless the athlete is wearing plastics (conditioning suit), which is a big no no anyway. If I remember correctly the body can take up to three days without water (everything being normal). I would imagine that most coaches that use this practice are trying to keep their athletes on the task at hand and not waste time with unneeded water breaks. It might sound a little brutal, but until you see how tough it is to keep kids focused for two hours in a wrestling room you probably wouldn't understand. Anyway, ask the coach about the practice of this I'm sure he would be happy to share the logic of it with you.