You can drink water before your yoga class, but it isn't recommended to drink large amounts immediately before partaking in a session. By drinking water a couple of hours before your session, your body has time to absorb the liquid and keep you hydrated. You should then aim to drink one glass of water 15-30 minutes before your yoga session starts. Refrain from drinking during the class where possible as this can divert your body's attention away from the asanas you're practising and into digesting the water instead, and this can give an uncomfortable full feeling during your flow.