The answer to your question can be found in Ralph's conversation with Piggy when the two are discussing swimming. While Piggy reveals that he can't swim because of his asthma, Ralph announces that his father taught him to swim when he was five years old and that his father is a commander in the navy. He seems to have absolute confidence that "when [his father] gets leave," he will come and rescue the marooned boys.
Piggy, clearly not wanting to reveal details about his own parents, asks Ralph when this rescue will take place, to which Ralph replies, "soon as he can." His confidence in his father's ability to rescue them cannot be shaken, even when Piggy asks how his father would know the boys' whereabouts.
We get more detail about Ralph's father when his proud son brings him up again in chapter 2. He explains that the navy will find them because, according to his father, "there aren't any unknown islands left." With every island being mapped and labeled in the Queen's "big room full of maps," Ralph shares his absolute confidence that rescue is inevitable. He tells the boys that it may even be his father's ship that arrives to rescue them.
It seems that his father's position helps Ralph to gain the respect of his peers, as the boys begin to clap and Piggy shows his "open admiration" for Ralph. As a good leader, Ralph then points out that in order to help his father, the navy, or other rescuers to find them, they need a fire with smoke on the top of the mountain.