Janet Rosen

Walter's advice is good.
Where I currently train and where I trained just before that both permit water bottles on the periphery of the mat and etiquette assumes adults will drink or toilet as they need to :-)
The thing about water: the latest info I've read is that you don't want to continuously sip, as it just sits in your stomach. Take a good deep drink.
Personally, when life permits (which is usually...) I like to eat a small balanced meal about an hour and a half to two hours before training (protein, some fat, usually carbs in the form of veg or fruit), and then drink a bunch of water or unsweetened iced tea (or hot tea in winter) between a half hour and an hour before training.
My practical rule of thumb on dehydration is: if you need to pee every 2 to 3 hours and its not very concentrated (dark), you are ok. If low sodium/potassium is a concern because you are sweating a lot, I recommend salt and potassium tablets (or just pour salt and salt subsitute on your hand and lick it off, then drink water) because a lot of sports drinks that are full of sugar don't actually have that much electrolytes (if they did they'd be soooo unpalatable...)