The sub went missing while carrying five people to the wreckage of the Titanic.
At that depth, it’s not “slowly” filling up with water. As soon as the hull is breached, it would violently implode inside of a second.
It’s not a pleasant process, but a catastrophic failure would be ideal from a “way to go” standpoint.
By comparison the USS Thresher had a small brazing leak, followed by a freezing of the compressed air they tried to fill the ballast tanks with to emergency surface, and then got to experience the bulkheads imploding and slowly be crushed as they sank. There’s audio from nearby ships of it imploding online and it’s pretty terrifying to listen to.
This is an example of an accident of decompression:
Coward, Lucas, and Bergersen were exposed to the effects of explosive decompression and died in the positions indicated by the diagram. Investigation by forensic pathologists determined that Hellevik, being exposed to the highest pressure gradient and in the process of moving to secure the inner door, was forced through the crescent-shaped opening measuring 60 centimetres (24 in) long created by the jammed interior trunk door. With the escaping air and pressure, it included bisection of his thoracoabdominal cavity, which resulted in fragmentation of his body, followed by expulsion of all of the internal organs of his chest and abdomen, except the trachea and a section of small intestine, and of the thoracic spine. These were projected some distance, one section being found 10 metres (30 ft) vertically above the exterior pressure door.