We used to have that problem a lot, the chickens would drown in the water tanks we had for other livestock. Use a shallower water dish, one they can climb out of easily if they accidentally fall in. For my adult chickens, I use a shallow black rubber feed pan, the style popular for graining horses out of. It's about 12 inches across and 8 inches deep. (I like to keep a lot of water out for them, since it's hot here and they also have to share it with wildlife that sneaks in for a free drink--don't want the chickens to run out accidentally) For bantams or young birds, I'd use a very shallow dish, or one of those chick waterers that has a red (at least they used to be red, I assume they still are) base and a plastic jar that holds the water to dispense it into the base a little at a time. Feed stores should carry them.
