if we were to 'sink'.... as long as we dont sink til 'crush depth' (the depth at which the sub would implode) we would be able to be rescued. they would do it by sending out a diving bell to extract the crew in groups.
they would manually send this thing down to us from the surface, it would mate to our hatch, and groups of guys can go in there. it is also pressurized to regular atmosphere so that again you dont succumb to decompression sickness etc