Why do the boys make having fresh water in the huts a rule?

Ralph understands straight away that if the boys are going to survive on the island they need to follow some basic rules. One of those rules relates to the gathering and storage of fresh water. A regular source of good, clean water has been found in a nearby stream. Drinking directly from the stream is perfectly fine, but Ralph thinks it would be much more convenient to fetch water from the stream in coconut shells and store the water in huts. That way, when someone needs a drink they don't have far to go.
Although it seems like a good idea on paper, it doesn't work in practice because most of the boys are too lazy to fetch water. This is one of the many important responsibilities—another one would be the maintaining of the signal fire—that is neglected by the boys, and this leads to growing tensions between Ralph and Jack's gang of savages.