[desertsolitaire]

Multiple possibilities.
Vehicle catastrophically failed at the beginning of losing contact and imploded.
Vehicle lost power and either is sitting at bottom and hard to see or floated back to surface and is hard to see.
Supposedly it had fail-safes to ensure it returned to the surface even if power lost by would be hard to see there, in the open rough ocean, as it rides low in the water.
Entangled in debris down there, now out of oxygen.
Sad. Prayers for their souls and for the grieving families.
If the craft is ever found intact, perhaps they survived long enough to record what happened and leave messages to their families behind in the craft before they all died.

[lodi90]

Given reported QC issues it was probably a crack in the hull and instant death for all aboard.

[Red Badger]

1. Vehicle catastrophically failed at the beginning of losing contact and imploded. This. ........................

[1Old Pro]

If the craft is ever found intact, perhaps they survived long enough to record what happened and leave messages to their families behind in the craft before they all died.

Yep, that would be expected I suppose.