Should coaches control their players water time?
Yes, if you mean control to make sure players are properly hydrated. No, if you mean cutting that time short to "toughen players up".
Not unless it is a ridiculously long time.
I understand that some coaches are lunatics and have caused players to faint. But the vast majority give their players as much as needed. If players allowed their water time they would constantly skip out on practice giving a excuse, another example of how week this generation really is. If your coach is crazy, report it to the league founder (you won't be the only one btw.) and he will be removed from activists and coaching opportunities with the team.