The movie, “USS Indianapolis” staring Nicolas Cage is on Netflix.
The Indianapolis was never outfitted with sonar. A heavy cruiser’s role was to duke it out with other large and armored surface vessels, not to engage in anti-submarine warfare. Doctrine of the time was that a heavy cruiser would never put to sea without a full compliment of support vessels, such as torpedo destroyers, which DID have sonar and when necessary would engage submarines in defense of the cruiser. So the Indianapolis never got sonar because IN THEORY she would never have need it it.
But for the voyage home from Tinian, Indianapolis was denied a destroyer escort because destroyers were needed at Okinawa to rescue downed B-29 crews engaged in the bombing of mainland Japan. This despite the fact that a Japanese submarine had sunk a US torpedo destroyer along her intended route just six days before the Indianapolis was to set sail.
And despite the certain knowledge that Japanese subs were active in the area, all Capt. McVay received was a generic “possible enemy submarine activity” report, not that actual attacks had happened just days before, because they feared compromising security if they told him anything more specific. His “need to know” was not great enough.
So the Indianapolis was sent out alone, lacking any means to detect the presence of a submerged submarine, directly into known Japanese submarine activity, and with incomplete intelligence which served to downplay the high risk. Then when the foreseeable came to pass and a submarine sank his ship, Capt. McVay was court-martialed for “failing to zig-zag.” The US Navy lost 380 ships in combat during the Second World War and Charles McVay was the only captain to be court-martialed for losing his vessel.
The Indianapolis survivors organization put forward the theory that the severe punishment of the captain was meant to obscure the fact that the navy had failed to acknowledge the ship was overdue in a timely fashion, which delayed rescue efforts, which dramatically increased the loss of life.
On order of the US Congress, in 2001, SecNav purged Capt. McVay’s record but he had killed himself 33 years earlier.