There’s a comment on the Reddit that said James Cameron, who is apparently the Titanic expert, said communication within the Titanic community said they dropped their descent weights because there was a problem, were ascending back up, and they imploded on the ascension. He said there were alarms that went off inside to indicate the submersible was failing, and they probably knew something was wrong. So the death may have been instant, but they probably had that fear at the very end.
It’s somewhere in this interview (edit: 8:15)