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])