Well, i'm not sure there is any such thing as keeping any chicken from pooping anywhere, but you might try elevating the waterer a little bit so that it isn't so accessible to their back sides.
Keeping the water dish about shoulder height to the chicks will help with not just the pooping, but also other things getting it dirty.
In the end, though, i wouldn't worry too much about the poop in the water. Although you want to try to keep it clean and change it out often, chickens' immune systems are strengthened by this sort of thing, and i don't think you'll see any adverse effects if you change it out each day.