If I remember correctly there was dense fog that night and the skipper was sure they were safe in the cover but a sub got real close and spotted them.

Got off of wiki - In July 1945, Indianapolis completed a top-secret high-speed trip to deliver uranium and other components for "Little Boy", the first of two nuclear weapons ever used in combat, to the Tinian Naval Base, and subsequently departed for the Philippines on training duty. At 0015 on 30 July, the ship was torpedoed by the Imperial Japanese Navy submarine I-58, and sank in 12 minutes. Of 1,195 crewmen aboard, approximately 300 went down with the ship.[4] The remaining 890 faced exposure, dehydration, saltwater poisoning, and shark attacks while stranded in the open ocean with few lifeboats and almost no food or water. The Navy only learned of the sinking four days later, when survivors were spotted by the crew of a PV-1 Ventura on routine patrol. A PBY seaplane (which the USN had at the time) crew landed to save those in the water. Only 316 survived.[4]