Dark Dawn by Henry Kuttner — Read

The Albacore was eight hundred miles out of Suva, feeling her way through the Pacific toward a destination unmarked except on the charts. She was a Navy cruiser jury-rigged into a floating laboratory, Navy manned, but carrying a dozen specialized technicians as passengers.

For days she had waited outside the danger area, till circling planes radioed word that the test atomic blast had apparently subsided. Then the Albacore went into a flurry of preparations. It was a miracle that the watch had sighted Gresham in his rubber boat, and a triple miracle that he was alive.