The implosion of the Titan submarine (OceanGate) was so rapid that the pain wasn’t even registered by the brain. They didn’t feel anything at all. Utterly instantaneous.

Correct.

Using standard fluid dynamics equations (Bernoulli) to determine inrush velocity, water would accelerate into the void of the submersible at approximately 260 meters per second. Given the approximate size of the sub, the time to close the void would be around 2.7ms.

Standard minimum neural response time to any stimuli to have it register consciously is ~100ms.

Those people were all dead (converted to goo, essentially) long before they even realized anything had occurred.