What does Ralph see as he, Piggy, and Maurice play near the pool?
The episode that you are talking about happens in Chapter 4. In this part, Ralph sees (or at least he thinks he sees) a ship. He thinks he sees the smoke from a passing ship, but none of the others can see it.
This is most relevant because of what is going on with the signal fire. The boys think things will be good because the ship will see the fire. It is at that point that they realize the fire has gone out.
It went out because the choir was out hunting instead of taking care of it. This is a big turning point in the story -- it represents how the choir and Jack are becoming less responsible and civilized and are turning against Ralph and his attempts to keep order and a sense of purpose.