The boys get their water from a river that runs through the island. In chapter 5, page 91 in my edition, Ralph is speaking at the assembly after the fire has been left and extinguised, Golding writes:
We decide things. But they don't get done. We were going to have water brought from the stream and left in those coconut shells under fresh leaves. So it was, for a few days. No there's no water. The shells are dry. People drink from the river.
Lord of the Flies examines the potential of a group of boys, stranded on an island after their plane crashes and there are no adult survivors. Ralph and Piggy are the first boys to emerge and it is Piggy's initiative which allows them to establish the whereabouts of other survivors by blowing into a conch shell they discover.
Piggy measures everything against what his auntie might do or think and is clearly the intelligent one in the group. Ralph has a "stillness" (ch 1) about him which gives the boys confidence and the fact that he is holding the conch is what confirms his place as leader. It is Piggy who will help Ralph in his decision-making after Ralph is voted as leader because he has the capacity to reason. Now that there is a leader, the need to build shelters, find food and a good source of fresh water and build a rescue fire are the first things on Ralph's agenda. The boys start off with good intentions but with no "grown ups" they soon lapse in keeping to the rules.
The boys promise to bring fresh water to the camp from the stream leaving the water in coconut shells for drinking but they only do this for a few days, and the shells are now dry and the boys must drink from the river. Ralph calls a meeting, but not for "fun, but business" (ch 5) where they should discuss this problem of fresh water and all the other problems. Drinking from the river is fine but Ralph also feels that there should be water kept in the camp.