Ralph's father works in the navy. On page 13 in my edition we see that Ralph drops this into his conversation with Piggy, perhaps slightly boasting, to answer Piggy's question about how he learnt to swim so well:
"I could swim when I was five. Daddy taught me. He's a commander in the Navy. When he gets leave he'll come and rescue us..."
This perhaps foreshadows Ralph's leadership at the beginning of the novel, and also his role as the representative of civilisation and order in this work. We can also see his youthful optimism at a quick rescue, his dependence on "grown ups" and his inability to see that this experience is not something that they will be rescued from quickly.
In chapter one there are two places that show what Ralph's father did. In the beginning when Ralph is talking with Piggy, Ralph says that his father, who is a commander in the navy, taught him how to swim.
Ralph paddled backwards down the slope, immersed his mouth and blew a jet of water into the air. Then he lifted his chin and spoke. “I could swim when I was five. Daddy taught me. He’s a commander in the Navy. When he gets leave he’ll come and rescue us. What’s your father?”
Later in the chapter, as Ralph holds the conch shell, he encourages the boys by telling them that help is on the way. He speaks with authority, because his father was a navy man. Moreover, his father told him that there are no unknown islands left.
“My father’s in the Navy. He said there aren’t any unknown islands left. He says the Queen has a big room full of maps and all the islands in the world are drawn there. So the Queen’s got a picture of this island.”