For them, based on the report, it is implosion that killed them instantly. So for them, it is like "ooh, the ocean is so dark down here..." then bam! No more consciousness since they are compressed within 0.03 seconds, surely crushing their brains and bodies including that teenager, instantly and turning them into ocean mush for the bottom dwellers to feed. They did ont even feel being flash fried by the heat generated by the implosion, which is briefly as hot as the surface of the sun, which will surely kill any living thing not miles away from that sub.

But of course, dead men and boys don't tell no tales, so we cannot be certain. The scenario above is a rough estimate based on empirical data on the materials of the sub, the depth they are in upon implosion and the simulations of scientists on that situation.