On a drizzly August morning, Caroline Kennedy waded into the turquoise waters between two deserted islands in the South Pacific, trying not to scratch her feet on sprouts of coral.

“Look how beautiful this is,” she said.

“Your father did this swim,” said her son, Jack Schlossberg.

Together they stood in the Western Province of the Solomon Islands, facing a mile-long jaunt to an islet called Olasana — a place John F. Kennedy, Caroline’s father, landed almost exactly 80 years earlier as a junior Navy officer. He crossed the same waters to save himself and 10 members of his crew after a Japanese destroyer split their torpedo boat, PT-109, in the predawn darkness of Aug. 2, 1943.