Coastwatcher by Elise Weston

During World War II, the war was being fought in Europe, Africa and in Asia. But those who lived along the coast of the United States kept watch to be sure that the enemy was not sneaking into the country. One of those who voluntarily keep watch is Hugh. Even though his sister has lost interest in watching and his cousin Tom doesn't seem to want to help, Hugh spends part of each day with his binoculars staring out at the waters near his South Carolina home. One day he sees it. Out in the water. He is sure it is a periscope from a submarine. But no one will believe him. Has he just been spending so much time watching that he is imaging this?