One of the instructors at my yoga studio talks about how it's been years since he's had water with him in a Bikram class. Today was actually my third class water-less. And by water-less, I mean that I brought water in (just in case), but didn't actually drink any.
Bikram legend has it that if you've come into class dehydrated, it's already too late to rehydrate yourself with just water. It's why proper hydration throughout the day prior to a class or the night before if you're taking it in the morning is so important.
The temperature and humidity conditions of a Bikram yoga room are such that you're supposed to sweat, and you're supposed to sweat a lot. If you're improperly hydrated, your body doesn't have enough water to generate an adequate amount of sweat -- your body's built in cooling system can't work the way it's supposed to.
What that all boils down to is: that water you're drinking during Bikram class is more for reassuring you mentally, not so much physiologically.
I've found that my practice without water is actually a bit easier -- instead of drinking, I take the full 20 seconds allotted to me for savasana and calm my mind. I don't think about whether or not I'm going to drink, which means I'm more able to focus on my practice.
Obviously, I'm not a professional and I'm not advocating that everyone should stop drinking water while in physically demanding conditions. What I am saying is that with proper hydration, and listening to your body, it is definitely possible to complete the full 90 minutes without water -- as long as you make sure to rehydrate after class :)
Anyone else practice Bikram without water? What did you think?