Do you watch her 24 hours a day? Broodies tend to be sneaky when they get off the nest because they don't like anyone or anything to know where their nest is. They also don't leave the nest for long. Maybe 10-15 minutes at most. Chickens have been hatching eggs since the dawn of time without human intervention. If they weren't capable of taking care of themselves while on the nest, they'd be long extinct by now. If you're concerned about her drinking, can you move water closer to her nest? I wouldn't put it where she can drink without getting off the nest, though. I'd just put it close by where she can see it. And has to get off the nest to drink.