If the sub didn’t implode and it’s stuck down there, it’s gotta be hellish for the passengers.
Little light, not much water or food and slowly declining levels of oxygen, tight quarters with little room to move, etc.
How awful!
Don’t forget cold.
Sum of all fears...like being buried alive.
Plus the heat. You might think it would be freezing on the bottom of the ocean but remember the gas laws. Lots of pressure on a fixed volume of gas = lots of heat too.
They will die of hypoxia - lack of oxygen. NOT painful.
You get sleepy, foggy, go to sleep.
Then die.