Not a submariner, unless you count the ride at Disneyland back in the '60's. Have done crash and accident photography for autos and high-performance aircraft. Implosion would be crushing their bones among all the other damages. Including their skulls. Not exactly painless but very quick.Been there, done that, still have a few t-shirts.
The first boat I served on was the same class as the USS Thresher (SSN-593). The loss of the Thresher and "lessons learned" was part of our training and culture.
As I posted earlier, it is my opinion that the vessel suffered a catastrophic implosion instantaneously killing the occupants and the force of the implosion would have shredded the submersible and scattered it like so much confetti across the sea floor.
The implosive breach of the pressure hull would have resulted an instantaneous diesel effect inside the craft, incinerating and nearly simultaneously crushing the occupants. While it seems a particularly grizzly way to die they wouldn't have had a chance to say WTF?