OceanGates Titan Submersible is, for as much as they charge, mostly cobbled together.

[dclarston13]

9. The vessel can withstand the depth

Thats not the issue. Its like shooting a rocket into space can it get there? probably, but can it get back???

The original submarine that found the titanic was the Alvin it had multiple layers of protection to ensure the safety of the occupants. For example the bolts attaching the hatch were fitted with explosives that could be activated to open the hatch in an emergency. This is not rare, actually even the Buick TourX car has exploding bolts built into the hood hinges to protect a pedestrian should you hit one. The bolts blow the hood towards the pedestrian and protect then from going through the windshield. I think this feature is common on some European cars.
So Oceangate CHOSE not to incorporate this feature, Also the ballast (which is what makes the vessel dense enough to sink) on the Alvin was attached with dissolving attachments (Anodes) that if it was underwater for a predetermined amount of time they would fall off and the vessel would float to the surface.

To me this whole situation is similar to climbing Everest only much more dangerous. While climbing Everest there are times where if you get in trouble you are done. It seems to me like Oceangate chose not use a Sherpa to climb Everest.
Its a big ocean I have personally sailed across the Atlantic more times than I can count, for most of the trip you are your own. I sincerely hope they find the vessel in time, but time is running out. Also people use breathing air much faster when they are under duress or panic.