re: Titanic tourist submarine goes missing

Most positive situation would be loss of communication and the submersible has surfaced and is adrift far from the ship. A very bad but survivable situation is loss of coms on bottom and unable to ascend. There might be a few days of emergency air.

If rescue had been prearranged it would just be a matter of waiting for the calvary to arrive. Put a line on the stricken sub and hoist it. If not prearranged another sub or ROV and a ship with all the needed capabilities must be found that can reach the location during acceptable weather before the air is gone. Hypothermia is a possible factor; it's cold down there.