There is a tendency these days to clutch a water bottle whenever we are going to exert ourselves. The reason to refrain (if possible) from drinking during yoga is that you are quenching the essential thing that you want to generate: heat! When heat is produced internally through asana and the breath (rather than externally, as in saunas), we are purifying and cleansing the body and the mind. Because yoga works through the pranic (energetic), not just the muscoskeletal system, water during class impedes the energy flow. Whenever you put a substance into your body, energy is drawn to either digest or to warm the substance to body temperature, taking the energy away from where we want it: in our muscles. So, hydrate well before and after class instead of during class.