Ducks in Chicken Flocks: Environmental Health Disasters?

Hi all,

Is it a good idea to raise ducks and chickens togeather? It seems like chickens benefit from having a dry environment, whereas ducks like to turn things into muddy messes. Also, I've heard many times that chickens need very clean water. Why is this exactly? Is it important for the water to be clean just for purposes of general hygiene, or is it because chickens avoid water that doesn't look ro taste "right"? I'm asking because I have a flock composed of chickens and a duck, and the duck really likes to put soil in the communal water dishes. The water is changed every day, and is "clean" in that it doesn't seem to get feces in it. The chickens and the duck were raised togeather, and it doesn't seem to bother the chickens to have soil in their water- they drink from it regardless, even though they have access to clean water that is unaffected by the duck. However, my roommates think that  we should get rid of the duck because she is dirtying the water and therefore creating a sub-optimal environment for the chickens, perhaps causing them to not lay as much as they would normally (when we purchased the flock a month ago, we were told that each of the seven chickens would lay an egg a day, but normally we only get one or two). What do you all think? Does the duck threaten the environmental health of the chicken flock?

Thanks!