In Ashtanga yoga, its pretty typical for students not to drink water during the practice. Of course, if you need to, then drink the water. I drink water through out the day in order to be prepared for yoga practice, and to hydrate myself. I personally stop drinking water the hour before class to prevent feeling ill.
"It is very important to drink 10-12 glasses of water throughout the day so that you are well hydrated by the time you arrive at class. If you've been drinking all day, you shouldn't need to drink during class."