Realising something intellectually and experiencing it is two different things. Prisoners being waterboarded probably realise they are being tortured not killed, but it doesn't stop the terror. None of them can swim. I have a morbid fear of drowning so I'm less optimistic about the success of getting every single one of them out alive if they're going to be "swum" out. At least one section of the cave through which they must pass is wholly flooded. As well, think of the impact on those who remain as their friends depart, one by one. Apparently there are two boys and the coach who are weaker than the rest because of the days with little or no food, whose chances of surviving a diving experience is much lower