If the sub imploded...no one on board would have ever known what happened.
The human brain cannot perceive anything in less than four tenths (.4) of a second. An implosion is virtually instantaneous...meaning a human being in that situation would know and feel nothing. It is a painless and merciful way to die.
As a visual reference, here is a railroad tanker car imploding at ONE atmosphere of differential pressure. If they were at Titanic’s depth, they would have been at almost 400 atmospheres of DP across the hull. The implosion would have been that much faster.
Tanker car implosion
https://www.youtube.com/watch?v=Zz95_VvTxZM
If an implosion happened, as many are thinking, then it was indeed a quick, painless, and merciful way to die. Unfortunate for the family and friends, but they should take comfort in knowing that it was super fast. I know I would.