To sign water, take your dominant hand and extend and separate the three middle fingers while holding the thumb and pinkie finger together. Then tap your index finger on your chin a couple of times.
To remember the sign, it is simply the ASL sign for 'W', going near your mouth.
Once your baby is drinking fluids other than milk, start to teach signs associated with different liquids, so that she can indicate preferences when she is feeling fussy.
We use water in a variety of contexts from drinking to bathing, raining, or going to the pool. Water is one of the first signs your baby will learn.