Yeah, everyone calculating oxygen rates is a nice optimistic view on the situation.

I think occams razor suggests that they experienced a breach or defect in the viewing window (or possibly the hull).

At the pressures they were at, it would have been over for them, before they ever even knew there was a problem. The pressures are so great, that they would have assimilated into the water instantly.

For what its worth, as sad as that sounds, it is possibly a better fate than the alternative of suffocating to death for 4 days in complete darkness (which is exactly what happened to 23 crew members aboard the Kursk russian sub back in 2000 [1])

[1] https://en.wikipedia.org/wiki/Kursk_submarine_disaster

I also thought they suffocated but it seems more likely that they died in a fire.

"Analysts concluded that 23 sailors took refuge in the small ninth compartment and survived for more than six hours. When oxygen ran low, they attempted to replace a potassium superoxide chemical oxygen cartridge, but it fell into the oily sea water and exploded on contact. The resulting fire killed several crew members and triggered a flash fire that consumed the remaining oxygen, suffocating the remaining survivors."

Apparently they were trying to fix the cartridge and the officer lost control of it and was killed instantly, some of the others dove under the water and survived the fire, but as the fire had consumed all the oxygen, it didn't matter. How they figured all that out from what they hauled up, I can only imagine.